title: What does ~> mean in a gem file
body: <p>In the gem file for <a href="https://github.com/justinfrench/formtastic">https://github.com/justinfrench/formtastic</a> they have:</p>
  
  <pre><code> gem 'formtastic', '~&gt; 2.0.0'
  </code></pre>
  
  <p>What does the ~> mean.
  It actually gives me the error "Could not find gem 'formtastic (~> 2.0.0, runtime)' in any of the gem sources."</p>