Added a test app and Travis-CI.

I am working on a more systematic way to update grape-swagger-rails, so first we need tests :)

This adds a test app that has some basic integration tests that make sure SwaggerUI loads when you include this gem. I couldn't figure out how to transform the rails engine app into something testable, so I think this is the way to go. Please let me know if you think otherwise - you can run test app from `test-app` and do development this way, which is nice. 

You can see a successful build in https://travis-ci.org/dblock/grape-swagger-rails/builds/34195365.

You should enable Travis-CI for the project on https://travis-ci.org/profile/ first before merging this to get a green build.