Create script to handle heroku deploy notifications

[patcon]

Right now, we're using heroku's IRC deploy hook: gratipay/gratipay.com#2301

Using the http webhook and pointing it at a hubot route would make it so that the full payload was delivered, and the message would be set in one place, rather than in each apps envvars. This would also allow us to link to the github commit or the "compare" page for the diff between the last deploy, which isn't currently possible, as heroku msg templates don't have a mapping for the github repo :)

[patcon]

Done and done: https://github.com/patcon/hubot-heroku-deploy-notifier

Deployed in v1.7.0...v1.7.1