Updates Gemfile and database config to support deploying to heroku
Adds config for running under Puma on heroku
Adds heroku config for CD with Travis
Applies Figaro config to heroku deployments

The Figaro config file is encrypted, and Travis config is updated
to decrypt the encrypted file as part of the build process and
run the `figaro heroku:set` command in order to set environment
variables on the heroku dyno.