Added more ruby versions to travis

[iainbeeston]

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]

:+1: thanks a lot