add support for min_pool_size
add support for min_pool_size

agriffis/heroku-buildpack-pgbouncer,BetterWorks/heroku-buildpack-pgbouncer,realtymaps/heroku-buildpack-pgbouncer,Yobota/heroku-buildpack-pgbouncer,rmoorman/heroku-buildpack-pgbouncer,heroku/heroku-buildpack-pgbouncer