Enable `gzip_vary` for Nginx
Enable `gzip_vary` for Nginx

Instructs Nginx to send a `Vary: Accept-Encoding` header when returning
GZIPed content. Without this, the new CDN was sending cached GZIP content to
clients that didn't request it, resulting in garbage.

http://wiki.nginx.org/HttpGzipModule#gzip_vary

alphagov/govuk_mirror-puppet,alphagov/govuk_mirror-puppet,alphagov/govuk_mirror-puppet