Lock to Postgres 11 for Pkgr/Fix issues with postgres 12

[rbclark]

Currently installing this version of Heimdall by following the instructions in the README is broken. Since Postgres has released V12, we need to either lock the Pkgr heimdall file to use postgres11-server or fix all of the issues with V12. Specifically, there is currently an issue during heimdall installation which will cause a ActiveRecord::StatementInvalid: PG::UndefinedObject: ERROR: type "jsonb" does not exist error to be raised. Unsure what additional issues will be encountered beyond this error.