use Postgres db on Heroku config

[xcesco]

H2 does not work on Heroku platform. Migrate to Postgres database. There is a free setup

• https://stackoverflow.com/questions/53555422/how-is-spring-boot-aware-of-herokus-database-url-config-var
• https://devcenter.heroku.com/articles/heroku-postgres-plans
• https://devcenter.heroku.com/articles/heroku-postgresql#connecting-in-java