Rails 4.x support

https://trello.com/c/0IENOmZH/235-rails-4-1-support
https://trello.com/c/oTepa1rR/246-sync-enova-landable-with-trogdor-landable

- [x] Support using Landable with Rails 4.0.x and 4.1.x
- [x] Test that on Travis