title: Unable to install curb gem
body: <p>It gives me the error of</p>
  
  <pre><code> gem install curb
  Building native extensions.  This could take a while...
  ERROR:  Error installing curb:
      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:1:in `&lt;main&gt;'
  
  
  Gem files will remain installed in /home/user1/.rvm/gems/ruby-1.9.3-p392/gems/curb-0.8.3 for inspection.
  Results logged to /home/user1/.rvm/gems/ruby-1.9.3-p392/gems/curb-0.8.3/ext/gem_make.out
  </code></pre>
  
  <p><strong>rvm list</strong></p>
  
  <pre><code>rvm rubies
  
     ruby-1.9.3-p385 [ broken ]
  =* ruby-1.9.3-p392 [ x86_64 ]
     ruby-2.0.0-p0 [ x86_64 ]
  
  # =&gt; - current
  # =* - current &amp;&amp; default
  #  * - default
  
  
  user1@ubuntu:~$ sudo apt-get install libcurl3 libcurl3-gnutls libcurl4-openssl-dev
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  libcurl3-gnutls is already the newest version.
  libcurl4-openssl-dev is already the newest version.
  libcurl3 is already the newest version.
  0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
  </code></pre>
  
  <p>How do I fix it?</p>