Configure Travis to test Ruby 2.0.0 and 2.1.2

To ensure that we support Ruby versions other than 1.9.3.

I've chosen 2.1.2 as it's the latest stable release. 2.0.0 is the
previous stable release.
Merge pull request #86 from gds-operations/73308952-add-ruby-versions

[#73308952] Configure Travis to test Ruby 2.0.0 and 2.1.2