title: Dependency on either AR 3.2 or AR 4 in gemspec
body: <p>I'm working on a gem which works on both Rails 3.2 and Rails 4. The gemspec dependency currently looks like this:</p>
  
  <pre><code>spec.add_runtime_dependency 'activerecord', '&gt;= 3.2.0', '&lt; 4.1.0'
  </code></pre>
  
  <p>Is there a way to use the twiddle-wakka for this, most notably because the above would include a Rails 3.3 which I would have to test first. What I mean is sort of the union of the following two dependencies, however, only requiring either of the two:</p>
  
  <pre><code>spec.add_runtime_dependency 'activerecord', '~&gt; 3.2'
  spec.add_runtime_dependency 'activerecord', '~&gt; 4.0'
  </code></pre>