title: mysql2 Gem::Ext::BuildError: ERROR: Failed to build gem native extension
body: <p>When I download the rails application from github
  <a href="https://github.com/ari/jobsworth" rel="nofollow noreferrer">https://github.com/ari/jobsworth</a>
  An then give <code>bundle install</code> it throws error:</p>
  
  <pre><code>Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  An error occurred while installing mysql2 (0.4.4), and Bundler cannot continue.
  Make sure that `gem install mysql2 -v '0.4.4'` succeeds before bundling.
  </code></pre>
  
  <p>After I give the <code>sudo gem install mysql2 -v '0.4.4'</code> and it displays like:</p>
  
  <pre><code>Fetching: mysql2-0.4.4.gem (100%)
  Building native extensions.  This could take a while...
  ERROR:  Error installing mysql2:
      ERROR: Failed to build gem native extension.
  
      current directory: /var/lib/gems/2.4.0/gems/mysql2-0.4.4/ext/mysql2
  /usr/bin/ruby2.4 -r ./siteconf20180212-10708-v1q8lj.rb extconf.rb
  mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
  
  extconf failed, exit code 1
  
  Gem files will remain installed in /var/lib/gems/2.4.0/gems/mysql2-0.4.4 for inspection.
  Results logged to /var/lib/gems/2.4.0/extensions/x86_64-linux/2.4.0/mysql2-0.4.4/gem_make.out
  </code></pre>
  
  <p>After run <code>bundle install</code> it throws same error.</p>