Shippable CI

I integrated Shippable as a replacement to executing "python test.py" when submitting a pull request.

Pull Request https://github.com/Instagram/python-instagram/pull/97 should be merged first as Shippable supports travis.yml and has been renamed as shippable.yml i.e. http://docs.shippable.com/en/latest/config.html#configuration

I omitted https://github.com/Shippable/sample_python_redis as there are no unit tests for sample_app.py yet.

Also the Shippable Badge will need to be amended to reflect the git branches of https://github.com/Instagram/python-instagram rather than my forked repository.

"python tests.py" output below (yes this would be redundant if this pull request is merged):
```
..........................
----------------------------------------------------------------------
Ran 26 tests in 0.381s

OK
```
I've completed the Contributor License Agreement ("CLA") too.