Publish head revision reference with built code so that appversion can be reported by HTTP

Result on http://ft-polyfill-service-qa.herokuapp.com/v1/__about:

![image](https://cloud.githubusercontent.com/assets/1735391/4519307/4e10848e-4cb6-11e4-9099-80ee0a5545c7.png)

Sam, what do you think of this as a pattern?  We'd presumably have to mirror this in Jenkins for an FTP2 deployment, but I'm increasingly thinking we should just stick with Heroku for the polyfill service, since it doesn't require any access to FT resources.