Fixes deployment to Heroku

`django-heroku-memcacheify` is also in `requirements.txt`, giving the following error

    -----> Installing dependencies using Pip (1.3.1)
       Double requirement given: django-heroku-memcacheify==0.4 (from -r requirements/base.txt (line 18)) (already in django-heroku-memcacheify>=0.4 (from -r requirements.txt (line 4)), name='django-heroku-memcacheify')
       Storing complete log in /app/.pip/pip.log