Added more ruby versions to travis
Added more ruby versions to travis

Right now tests are run using the following ruby versions:

* 1.9.3
* 2.0.0
* 2.2.0

But not:

* 2.1.x
* 2.2.1
* jruby
* rubinius

I've altered the travis config so it uses the latest release of ruby
1.9, 2.0, 2.1 and 2.2, as well as the latest jruby and rubinius
releases.

Might be worth merging #36 first

jorgemanrubia/mailgun_rails,rgraff/mailgun_rails,jorgemanrubia/mailgun_rails,rgraff/mailgun_rails,jorgemanrubia/mailgun_rails,rgraff/mailgun_rails