Updates Heroku deployment

Heroku has removed the user-env-compile plugin which we were using to access ENV vars in the Gemfile : https://devcenter.heroku.com/articles/labs-user-env-compile

Fortunately there is still a way to detect that bundle is running on Heroku because Heroku is injecting the stack name (ie "cedar") in the STACK env variable. When launching processes this env variable is not populated but the DYNO env variable is populated with the dyno name. So if one of these env variables is populated we should be running on Heroku.