fix(client): add "Referer" header to requests

When running Deis behind an SSL-terminated endpoint, there are
issues with Django's CSRF protection. Adding a Referer header
is mandatory when using SSL with Django.

Reference: https://code.djangoproject.com/ticket/16870

fixes #1002
Merge pull request #1218 from deis/1002-add_referer_header

fix(client): add "Referer" header to requests