MWEB-1060 Fixed newrelic agent for maps

After we switched to `gulp build` step and new deploytools plugin the service stopped sending data to New Relic. It seems the small change in `server/app.js` fixes the issue. I also added a little bit more comments about it ;)

Connected pull requests:
* https://github.com/Wikia/config/pull/715
* https://github.com/Wikia/chef-repo/pull/3428