Fix gitlab pages cache config

Merged Nick Thomas requested to merge fix-gitlab-pages-cache-config into master

The current Gitlab Pages NGINX configuration is broken for GitLab Pages v0.5.0+

Pages specifies a Cache-Control header, which NGINX tries to respect, but the default proxy_cache_key value is unsuitable as it doesn't take hostname into account. So, https://foo.pages.io/index.html and https://bar.pages.io/index.html are both cached under the same key.

We didn't expect NGINX to be caching this data, and it adds no value, so disable it.

This should be picked into %9.5

Closes https://gitlab.com/gitlab-org/gitlab-pages/issues/73

Edited by Nick Thomas