Update Ettu to work with Rails 4.1

Rails 4.1 is now supported on the 4.2 releases. (Confusing, I know.)

This ends direct support for Rails 4.0. The last compatible Ettu version
will be v4.1.8. More v4.1.* Ettu releases (not to be confused with Rails
v4.1.*) will only released to support breaking changes made by Rails
4.0.*.