[PR] Upgrade to Nginx 1.7.0 mainline release

For a good rundown of Nginx versioning, see http://nginx.com/blog/nginx-1-6-1-7-released/

1.7.0 is basically the same as 1.5.14. Nginx recommends:

<blockquote>In general, you should deploy the NGINX mainline branch at all times.  You may wish to use stable if you are concerned about possible impacts of new features, such as incompatibility with third-party modules or the introduction of bugs in new features.</blockquote>

If any bugs are introduced, we can roll back pretty quickly.