[#491] Pin every dependency's exact version.

To create these files, I've used pip-dump from the pip-tools package.

All requirements in *-requirements.txt have their versions pinned, and in
setup.py we keep only our direct dependencies and with the versions that
should work (i.e. >=0.11).

We did this so we can guarantee that everyone is using the exact same
version as everybody else, including our deployments.

For more info, check http://nvie.com/posts/pin-your-packages/.