当我尝试使用parallel_tests Gem并行运行我的规范时，我收到以下错误消息：
foo:~/Projects/cucumber-capybara-saucelabs/spec/requests [] ➜ rake parallel:spec
(in /Volumes/Storage/Projects/cucumber-capybara-saucelabs)
8 processes for 5 specs, ~ 0 specs per process
Starting Rails server on port 3001...
Starting Rails server on port 3001...
Starting Rails server on port 3001...
Starting Rails server on port 3001...
Starting Rails server on port 3001...
[2013-05-22 14:40:12] INFO WEBrick 1.3.1
[2013-05-22 14:40:12] INFO ruby 1.9.3 (2013-02-22) [x86_64-darwin12.3.0]
[2013-05-22 14:40:12] INFO WEBrick::HTTPServer#start: pid=33256 port=3001
Rails server running!
Rails server running!
Rails server running!
Rails server running!
[2013-05-22 14:40:12] INFO WEBrick 1.3.1
[2013-05-22 14:40:12] INFO WEBrick 1.3.1
[2013-05-22 14:40:12] INFO ruby 1.9.3 (2013-02-22) [x86_64-darwin12.3.0]
[2013-05-22 14:40:12] INFO WEBrick 1.3.1
[2013-05-22 14:40:12] INFO ruby 1.9.3 (2013-02-22) [x86_64-darwin12.3.0]
[2013-05-22 14:40:12] INFO ruby 1.9.3 (2013-02-22) [x86_64-darwin12.3.0]
[2013-05-22 14:40:12] WARN TCPServer Error: Address already in use - bind(2)
[2013-05-22 14:40:12] WARN TCPServer Error: Address already in use - bind(2)
[2013-05-22 14:40:12] WARN TCPServer Error: Address already in use - bind(2)
/Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `initialize': Address already in use - bind(2) (/Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `initialize': Address already in use - bind(2) (Errno::EADDRINUSE)
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `new'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `block in create_listeners'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:82:in `each'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:82:in `create_listeners'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/server.rb:82:in `listen'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/server.rb:70:in `initialize'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/httpserver.rb:45:in `initialize'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/handler/webrick.rb:10:in `new'
from /Users/foo/.rvm/Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/server.rb:323:in `delete': No such file or directory - /Volumes/Storage/Projects/cucumber-capybara-saucelabs/tmp/pids/server.pid (Errno::ENOENT)
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/server.rb:323:in `block in write_pid'
/Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `initialize': Address already in use - bind(2) (Errno::EADDRINUSE)
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `new'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:85:in `block in create_listeners'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:82:in `each'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/utils.rb:82:in `create_listeners'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/server.rb:82:in `listen'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/server.rb:70:in `initialize'
from /Users/foo/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/webrick/httpserver.rb:45:in `initialize'
from /Users/foo[2013-05-22 14:40:12] INFO WEBrick 1.3.1
sa/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/handler/webrick.rb:10:in `new'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/handler/webrick.rb:10:in `run'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/rack-1.4.5/lib/rack/server.rb:268:in `start'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/railties-3.2.13/lib/rails/commands/server.rb:70:in `start'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/railties-3.2.13/lib/rails/commands.rb:55:in `block in <top (required)>'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/railties-3.2.13/lib/rails/commands.rb:50:in `tap'
from /Users/foo/.rvm/gems/ruby-1.9.3-p392@rails-3/gems/railties-3.2.13/lib/rails/commands.rb:50:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
有没有办法避免这种情况？