setup travis CI testing

I cherry picked in https://github.com/dougbradbury/spree_braintree_cse/pull/4, and setup Travis CI. I separated the PRs so you can work with them individual if you want. If both changes look good, ditch #4 and go with this one.

Travis CI says the tests ar green, https://travis-ci.org/think602/spree_braintree_cse/builds/28184111. :)