title: sunspot_solr error on heroku?
body: <p>I'm using <code>'sunspot_rails', '~&gt; 2.0.0.pre.120415'</code> for Solr search in my rails app and deploy it on heroku (two separate application).
  It's working fine on one application but throw error on other.</p>
  
  <p>I have checked the heroku environment and found that -
  one app is on cedar stack and ruby version is 1.9.2p290. (Working fine)
  other one is on bamboo stack and ruby version is 1.9.2p180 (throw error)</p>
  
  <p>So, Is this the ruby version problem or heroku stack problem ?
  Ruby version on development environment is 1.9.2p290.</p>
  
  <p><strong>Error</strong></p>
  
  <pre><code>$ heroku run rake sunspot:reindex
  Running rake sunspot:reindex attached to terminal... up, run.8567
  rake aborted!
  uninitialized constant Sunspot::Adapters::Registry::Forwardable
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot/adapters.rb:310:in `&lt;class:Registry&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot/adapters.rb:309:in `&lt;module:Adapters&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot/adapters.rb:34:in `&lt;module:Sunspot&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot/adapters.rb:1:in `&lt;top (required)&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot.rb:18:in `block in &lt;top (required)&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot.rb:15:in `each'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot-2.0.0.pre.120925/lib/sunspot.rb:15:in `&lt;top (required)&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot_rails-2.0.0.pre.120925/lib/sunspot/rails.rb:1:in `&lt;top (required)&gt;'
  /app/.bundle/gems/ruby/1.9.1/gems/sunspot_rails-2.0.0.pre.120925/lib/sunspot_rails.rb:6:in `&lt;top (required)&gt;'
  /app/config/application.rb:7:in `&lt;top (required)&gt;'
  /app/Rakefile:4:in `require'
  /app/Rakefile:4:in `&lt;top (required)&gt;'
  (See full trace by running task with --trace)
  </code></pre>