title: cannot start rails server -> cannot load such file -- mysql2/mysql2 (LoadError)
body: <p>My rails application cannot start because mysql2 load error.
  I have tried many solutions, But none can't fix the problem.
  Even the rake command can't be executed.</p>
  <pre><code>rake aborted!
  LoadError: cannot load such file -- mysql2/mysql2
  </code></pre>
  <p>My gemfile is:</p>
  <pre><code>ruby '2.5.1'
  gem 'rails', '~&gt; 4.2.4'
  gem 'mysql2', '~&gt; 0.5.2'
  </code></pre>
  <p>Bundle installed successfully. Mysql installed correctly. Yet the error occurs on MacOS.</p>