Add Travis CI configuration.

[Our “builds” are failing](https://travis-ci.org/alexpearce/CharmProduction/builds/43354051) :wink: 

I've added [flake8](https://pypi.python.org/pypi/flake8), which mainly checks for [PEP8](https://www.python.org/dev/peps/pep-0008/) conformity. I find using it generally makes code more readable.

I've had some unit tests sitting locally that I would like to add at some point. Now the code is getting some structure I think at least some “sanity tests” would be helpful (e.g. the data loads, a fit runs).

Happy to hear your thoughts!