Update django-pylibmc dependency to >=0.6.1
Update django-pylibmc dependency to >=0.6.1

As a bonus, django-pylibmc 0.5.0+ supports Python 3, for which testing
will be enabled in another PR.

I've switched to using greater than, since the Python packaging
guidelines say it's not best practice to use `install_requires` to pin
dependencies to specific versions:
http://python-packaging-user-guide.readthedocs.org/en/latest/requirements/#install-requires

rdegges/django-heroku-memcacheify