我正在使用mac来启动daniel Kenoe的rails教程。我在开始教程的前几个步骤中遇到了一个问题。我无法连接到localhost：3000
附件是代码：
Micahs-MacBook-Air:learn-rails micahcrews$ rails s
=> Booting WEBrick
=> Rails 4.2.6 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
Exiting
/Users/micahcrews/workspace/learn-rails/config/initializers/simple_form.rb:2:in `<top (required)>': uninitialized constant SimpleForm (NameError)
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/engine.rb:652:in `block in load_config_initializer'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/activesupport-4.2.6/lib/active_support/notifications.rb:166:in `instrument'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/engine.rb:651:in `load_config_initializer'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/engine.rb:616:in `block (2 levels) in <class:Engine>'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/engine.rb:615:in `each'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/engine.rb:615:in `block in <class:Engine>'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:30:in `instance_exec'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:30:in `run'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:55:in `block in run_initializers'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:422:in `block (2 levels) in each_strongly_connected_component_from'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:431:in `each_strongly_connected_component_from'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:421:in `block in each_strongly_connected_component_from'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:44:in `each'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:44:in `tsort_each_child'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:415:in `call'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:415:in `each_strongly_connected_component_from'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:349:in `block in each_strongly_connected_component'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:347:in `each'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:347:in `call'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:347:in `each_strongly_connected_component'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:226:in `tsort_each'
from /Users/micahcrews/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:205:in `tsort_each'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/initializable.rb:54:in `run_initializers'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/application.rb:352:in `initialize!'
from /Users/micahcrews/workspace/learn-rails/config/environment.rb:5:in `<top (required)>'
from /Users/micahcrews/workspace/learn-rails/config.ru:3:in `require'
from /Users/micahcrews/workspace/learn-rails/config.ru:3:in `block in <main>'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/builder.rb:55:in `instance_eval'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/builder.rb:55:in `initialize'
from /Users/micahcrews/workspace/learn-rails/config.ru:in `new'
from /Users/micahcrews/workspace/learn-rails/config.ru:in `<main>'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/builder.rb:49:in `eval'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/builder.rb:49:in `new_from_string'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/builder.rb:40:in `parse_file'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/server.rb:299:in `build_app_and_options_from_config'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/server.rb:208:in `app'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/server.rb:61:in `app'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@learn-rails/gems/rack-1.6.4/lib/rack/server.rb:336:in `wrapped_app'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/server.rb:139:in `log_to_stdout'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/server.rb:78:in `start'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:80:in `block in server'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:75:in `tap'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:75:in `server'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
from /Users/micahcrews/.rvm/gems/ruby-2.3.0@global/gems/railties-4.2.6/lib/rails/commands.rb:17:in `<top (required)>'
from bin/rails:4:in `require'
from bin/rails:4:in `<main>'