basic travis integration

Hi, first of all thanks for your gem! You do a great job!

I've added a travis-ci.org config for you, so you can test against several ruby/rails versions. Currently all builds are passed (you can check it on https://travis-ci.org/sharpyfox/better_errors/builds/4313014).

Before(or after) merge you should enable test builds for this repository in travis-ci account setings: https://travis-ci.org/profile . Otherwise build status will always be "unknown"