Improve the Travis build

Hello,

This is just a tiny pull request that improves the Travis set-up in order to get better feedback:

* Remove unneeded dependencies for Rubinius from the different `Gemfile`s.
* Avoid testing against current Rails' master for Ruby older than 2.2.
* Add 2.2 inside the build matrix.
* Add a `Gemfile` for the `4-2-stable` branch.

Have a nice day!