title: Replacing RVM with rbenv --mkmf error
body: <p>I was looking at replacing RVM with rbenv since I am using bundle to manage my gemfiles. rbenv,  I believe, is a lot lighter in managing my Ruby versions, so I uninstalled RVM and installed rbenv. I installed the bundler gem using</p>
  
  <pre><code>gem install bundler
  </code></pre>
  
  <p>because we need it everywhere. Then I opened a project and installed the required version Ruby using rbenv:</p>
  
  <pre><code>rbenv install 1.9.3-p194
  rbenv local 1.9.3-p194
  rbenv rehash
  </code></pre>
  
  <p>I entered <code>ruby -v</code> to make sure it was using the right version of Ruby, and it was, so I tried running bundle and it ran until it tried to install RedCloth and and then failed:</p>
  
  <pre><code>Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
      /usr/bin/ruby1.9.1 extconf.rb --with-cflags=-w
      /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:1:in `&lt;main&gt;'
  </code></pre>
  
  <p>It suggested that I install RedCoth before bundling, so I installed RedCloth:</p>
  
  <pre><code>gem install RedCloth
  </code></pre>
  
  <p>which succeeded. I then ran <code>ruby -v</code>, just to make sure it's still right, then tried running <code>bundle update</code> and the same error occured: </p>
  
  <pre><code>Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
      /usr/bin/ruby1.9.1 extconf.rb --with-cflags=-w
      /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:1:in `&lt;main&gt;'
  
      Gem files will remain installed in /home/ruberto/.bundler/tmp/7551/gems/RedCloth-4.2.9 for inspection.
      Results logged to /home/ruberto/.bundler/tmp/7551/gems/RedCloth-4.2.9/ext/redcloth_scan/gem_make.out
      An error occurred while installing RedCloth (4.2.9), and Bundler cannot continue.
      Make sure that `gem install RedCloth -v '4.2.9'` succeeds before bundling.
  </code></pre>
  
  <p>I then tried installing a different version of Ruby but had no success there either.</p>
  
  <p>Does anyone have ideas?</p>
  
  <p>I subsequently tried doing the same thing in my other projects and it all seems to work just fine. I have one that uses 1.8.7 and it was as easy as setting the rbenv to the right version and then running bundle. I tried almost every combination of Ruby version and yet it's not working.</p>