Remove HEROKU mentions in the buildpack

[raphodn]

There is mentions of Heroku in at least 2 files : Procfile & bin/start
It could be a good idea to look into these files and remove the reference to Heroku, and adapt them to Scalingo ?
Probably linked to #5 which sets a MAX_METASPACE_SIZE that is not adapted to the default Scalingo one-click deploy button.