捆绑更新给出错误

Question

我在运行'捆绑安装'

时遇到以下错误

Bundler could not find compatible versions for gem "railties":

 In Gemfile:

 rails-api (>= 0) ruby depends on
  railties (>= 3.2.11) ruby

rails (= 3.2.9) ruby depends on
  railties (3.2.9)

而更新捆绑也会出现相同的错误

虽然gem install railties提供以下

ERROR:  Error installing railties:
ERROR: Failed to build gem native extension.

    /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:13:in `<main>'

我的宝石文件看起来像

source 'https://rubygems.org'

gem 'rails', '3.2.9'

gem 'sqlite3'

gem 'webapidoc', :github => 'otty/webapidoc'
#gem 'webapidoc', path: '/Users/lion/Work/rails/webapidoc'

gem 'rails-api', :github => 'rails-api/rails-api'

gem 'foreman'

gem 'tire'
gem 'yajl-ruby'
gem 'sync_attr'

gem 'thin'

gem 'capistrano'
gem 'capistrano-unicorn'
gem 'capistrano-ext'
gem 'rvm-capistrano'
gem 'capistrano-detect-migrations'