Update README to include correct key for Heroku
Update README to include correct key for Heroku

README was claiming that `GMAPS_API_KEY` was required to be set when it should be `GOOGLE_MAPS_API_KEY`.
transistorsoft/background-geolocation-console,transistorsoft/background-geolocation-console,transistorsoft/background-geolocation-console,transistorsoft/background-geolocation-console