added ruby 2.1 series to supported versions

I have just run the tests against ruby 2.1.1 and ruby 2.1.2 (did not against 2.1.0), accordingly with the tests it seems foreman is supported.
If there is nothing else you know we could include this line in the docs.
Update travis.yml to include ruby 2.1 series
Updated Readme to specify supported ruby 2.1 series
Merge pull request #481 from formigarafa/patch-1

added ruby 2.1 series to supported versions