Unpin installation requirements and get version without importing module

Pinned requirement versions may conflict with the installed project's requirements. Minimum requirements are specified, but a project's version of requests will not be downgraded if this module is later installed in the environment.

Importing the module in `setup.py` to read the `__version__` proved problematic because `census` depends on `requests` - and in a fresh installation this can't be expected to be installed already. By reading the file as a file and not a Python module this is avoided entirely. I encountered this problem trying to deploy a small app to Heroku and then duplicated trying to test with tox.