title: mri_21 is not a valid platform
body: <p>Hi i switched my Ruby on Rails development platform from Windows 7 to Linux (Debian), I copied entire app directory to Linux and run <code>bundle install</code>and the command was successful. Than i run <code>rake db:migrate</code> and it quit with following error:</p>
  
  <pre><code>`mri_21` is not a valid platform. The available options are: [:ruby, :ruby_18, :ruby_19, :mri, :mri_18, :mri_19, :rbx, :jruby, :mswin, :mingw, :mingw_18, :mingw_19]
  </code></pre>
  
  <p>Than i Googled for the error and found <a href="https://github.com/RailsApps/rails_apps_composer/issues/297" rel="nofollow">this</a>, i followed the suggested steps, i.e.:</p>
  
  <pre><code>gem update bundler
  </code></pre>
  
  <p>above command output:</p>
  
  <pre><code>Updating installed gems
  Nothing to update
  </code></pre>
  
  <p>Than: <code>bundle install</code> and this was also successful.
  But still getting same error on <code>rake db:migrate</code></p>
  
  <p>my ruby version: <code>ruby 1.9.3p194</code>
  unable to get rails version by <code>rails -v</code> getting the same mri_21 error but in my app's Gemfile i have <code>gem 'rails', '4.1.5'</code></p>
  
  <p>EDIT: But when i tried <code>bundle exec rake db:migrate</code> it was successful. and than i run <code>bundle exec rails s</code> and app running successfully :</p>
  
  <pre><code>=&gt; Booting Thin
  =&gt; Rails 4.1.5 application starting in development on 0.0.0.0:3000
  =&gt; Run `rails server -h` for more startup options
  =&gt; Notice: server is listening on all interfaces (0.0.0.0). Consider using 127.0.0.1 (--binding option)
  =&gt; Ctrl-C to shutdown server
  &gt;&gt; Thin web server (v1.3.1 codename Triple Espresso)
  &gt;&gt; Maximum connections set to 1024
  &gt;&gt; Listening on 0.0.0.0:3000, CTRL+C to stop
  </code></pre>
  
  <p>Now my question is, how to get rid of this <code>bundle exec</code> every time, and what's going on with this.</p>