Added ruby 2.2 to Travis configuration

Improved .travis.yml to include ruby `2.2` and changes `2.1.0` to `2.1`

I didn't add the patch level by purpose, because as fair as I understand, starting with 2.1 using X.Y is the same of using X.Y.Z in 2.0.0.

E.g:
https://travis-ci.org/diowa/twbs_sass_rails/jobs/44005317#L120
https://travis-ci.org/diowa/twbs_sass_rails/jobs/35764885#L46

Both the above builds use 2.2.0, but as you can see, the first time my gem was tested against `2.2.0-p598` and the latter against `2.2.0-p481`.

In this way, gem will be tested against `2.1.5` and `2.2.0`. When `2.2.1` will be released, there will be no need to update this configuration file

I don't know when it will be possible to test ruby 2.2 on travis, but tests pass on my machine