title: `bundle exec` doesn't work, says "`mri_22` is not a valid platform"
body: <p>I'm trying to run a command using <code>bundle exec</code> in my Ruby project. Whenever I run it, though, I get an error message that looks like:</p>
  
  <pre><code>$ bundle exec rake test
  `mri_22` is not a valid platform. The available options are: [:ruby, :ruby_18,
  :ruby_19, :ruby_20, :ruby_21, :mri, :mri_18, :mri_19, :mri_20, :mri_21, :rbx,
  :jruby, :jruby_18, :jruby_19, :mswin, :mingw, :mingw_18, :mingw_19, :mingw_20,
  :mingw_21, :x64_mingw, :x64_mingw_20, :x64_mingw_21]
  </code></pre>
  
  <p>How do I get <code>bundle</code> to work?</p>