Travis support ruby 2.1.0 rails 4.x

add more travis support:

- add ruby 2.1.5
- add gemfiles for 4.0.x, and 4.1.x
- use mintest
no minitest here, like https://github.com/code-mancers/ministry_of_state/pull/10/files#diff-edad3ce9327a1c46acbc9dbab8136112R7