add Travis, test with ruby 2.1

@jeremyevans Thanks for this gem!  It would be helpful to see build status for newer rubies, so this PR adds Travis testing.
 * Gemfile (so travis can build these)
 * Add travis-ci build badge and code climate.
 * Update tested ruby versions.

Note, I tested travis builds with my fork here: https://travis-ci.org/spovich/ruby-american_date
You will need to login to travis and create your profile to get the build badge link in this PR to work.