title: How to add dependency of a local gem to a rails plugin/engine, in .gemspec file
body: <p>I had tried in this way：</p>
  
  <pre><code> s.add_dependency 'gem', :path =&gt; '../gem'
  </code></pre>
  
  <p>like add gem in the <code>gemfile,</code> but it doesn't work, and will cause this error:</p>
  
  <pre><code>/Users/chenqh/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/requirement.rb:81:in `parse': Illformed requirement 
  </code></pre>