Rails 4.1 Support

1. Update Finders module to look up the correct class-specific ActiveRecord::AssociationRelation for both Rails 4.0.x and 4.1.x
2. Add Rails 4.1.x to tested gemfiles and .travis.yml
3. Temporarily allow failures on Rails 4.1 / JRuby until this issue - https://github.com/jruby/activerecord-jdbc-adapter/pull/523 - is addressed