title: Rails application using Postgres adapter can't activate pg
body: <p>In a Rails application, with a bare <code>pg</code> requirement in your Gemfile:</p>
  
  <pre><code>gem 'pg'
  </code></pre>
  
  <p>You'll get the following error:</p>
  
  <pre><code>Gem::LoadError can't activate pg (~&gt; 0.18), already activated pg-1.0.0. Make sure all dependencies are added to Gemfile.
  </code></pre>