On continuous integration

I have configured these two CI services:

Travis
Shippable

because hey allow to run on multiple versions of Python.

Patches are welcome to fix the failing cases!

Travis

Configuration in .travis.yml.

Results: https://travis-ci.org/AndreaCensi/contracts/

Shippable

Configuration in shippable.yml.

Results: https://app.shippable.com/github/AndreaCensi/contracts