Cache key could cause errors with memcached

[MarZab]

Got a warning:

/home/vagrant/env/lib/python3.4/site-packages/django/core/cache/backends/base.py:224: CacheKeyWarning: Cache key contains characters that will cause errors if used with memcached:

[martey]

Sorry for the lack of response on this.

If you wanted to submit a pull request for this, I would accept it.