Prohibit installation via Python2

[tushar-rishav]

Presently, the app has been tested with Python3.4. If a user tries to install via Python2, he gets no immediate warnings. It'd be nice to warn users if installation is being performed via Python2.
This can be implemented with simple check in manage.py maybe?

import sys
if not sys.version_info > (3, 3):
    print("Hey! We support Python 3.4 or later.")
    exit(13)

cc @nlhkabu

┆Issue is synchronized with this Asana task