Configure nginx to proxy http://jenkins.dev/

The `jenkins.conf` template was almost there. What remained was to add a `proxy_pass` directive for `location /`.

* proxies to upstream jenkins server
* configures [recommended proxy settings](http://www.cyberciti.biz/tips/using-nginx-as-reverse-proxy.html)
* upstream renamed from `jenkins.dev` → `jenkins` to make it clearer in the nginx config that it's an upstream and not the actual hostname
* fixes boxen/our-boxen#197

This is working on my Mac Mini using the our-boxen template with only `java` and `jenkins` modules added.