Modified requirements.txt to allow later versions of the keen package

The original requirements.txt fixed the Keen client to version 0.3.0.
Currently the Keen client is on version 0.3.3, so installing django-keen makes
keen revert to the previous version.
This commit accepts any version of keen >= 0.3.0
Merge pull request #1 from simpleigh/keen-version

Modified requirements.txt to allow later versions of the keen package