Dependency and testing updates

Hi,

Our team had trouble installing the older FFI 1.5 dependency. So we bumped the FFI dependency and did some cleanup here and there. We also integrated Travis CI to test 1.9.3 and 2.0.0 compatibility. (See the successful build [here](https://travis-ci.org/Spokeo/levenshtein-ffi/builds/7578532)).

Thanks, hope you'll accept this pull request and release a new minor version soon!