Add heroku deployment script & remove sentry-raven

Sentry should be removed from the project because it's not an essential gem. It's just a tool for getting notified about errors and helps you troubleshoot them. It's up to the redeployment team what debugging and reporting tools they want to use.
Merge pull request #341 from codeforamerica/heroku-deploy-script

Add heroku deployment script & remove sentry-raven