Correct AR version number in README
Correct AR version number in README

gregburek/heroku-buildpack-postgresql,rmoorman/heroku-buildpack-pgbouncer,heroku/heroku-buildpack-pgbouncer,Shyp/heroku-buildpack-pgbouncer,BetterWorks/heroku-buildpack-pgbouncer,musicglue/heroku-buildpack-postgresql,realtymaps/heroku-buildpack-pgbouncer,Yobota/heroku-buildpack-pgbouncer,agriffis/heroku-buildpack-pgbouncer