Add tests and run through Travis CI

[leonsodhi]

That would allow us to test new changes on multiple versions of Ruby and avoid situations where a method is called that doesn't exist. For example, here force_encoding doesn't exist on 1.8.7.

[DannyBen]

Is this on someone's near term agenda? It would make #78 a little easier in terms of confidence to make changes.

[troy]

@DannyBen No plans in the foreseeable future. We're blessed with lots of really high-impact ways to improve Papertrail. Even as someone who loves great coverage, #78 is a low-impact improvement for the amount of time it would take, and other than that change, the codebase doesn't change much. Based on that, this wouldn't be a good use of time right now (nor would #78, but we're perfectly willing to let someone refactor if it strikes their fancy).