This is a patch release fixing two bugs.

• Trying to run with sprockets-rails-3.0.0 would cause NoMethodError: undefined method 'register_engine' for nil:NilClass. To fix this we're locking the dependency of sprockets-rails to 2.x. (See #609 and #618)
• The new :image factory failed with no arguments when used in an application's test suite because the example image wasn't being included into the gem release.