fix(security/dj-secure): add missing middleware of django-secure

- For the working of django-secure the middleware class needs
to be added. (http://django-secure.readthedocs.org/en/latest/index.html#usage)
- add correct values for SECURE_PROXY_SSL_HEADER for heroku
Merge pull request #130 from theskumar/fix-dj-secure

fix(security/dj-secure): add missing middleware of django-secure