Replace Jenkins with Travis as recommended CI

As out old Jenkins server is getting old, and Jenkins itself isn't quite as smooth as silk, a replacement is needed. The DrFront team, AMP, and myself have all tested using a Travis Pro setup instead, and the results are promising. 

Main takeaways:

- SaaS, no need to manage our own installation
- VM based, meaning no dependencies are shared between repos, and testing with multiple PHP versions is trivial
- Fast, at least a lot faster than our old Jenkins setup
- Easier to configure
- Better integrated with other tools