Update travis ruby versions

[kitop]

Currently the gem is being tested against 1.9, 2.0, 2.1, and 2.2.

• 1.9 and 2.0 are end-of-life'd, which means they won't receive security fixes and thus we don't need to test against them.
• 2.3 should be added to the test matrix

The travis.yml file is the file to update.