Bump requests to latest version

[rn-c1]

The requests package in setup.py is really old, and it breaks compatibility with other packages that I would like to have installed in the same virtualenv.

This could be as easy as changing "requests==2.2.1", to "requests>=2.2.1", in setup.py.

[chuwy]

Good point. Let's make Python Tracker compatible to latest version in next release and pin it in setup.py.

Though if it is possible I would factor out as many dependencies from trackers as possible.