Gracefully handle PostgreSQL minor updates in Procfile

We had a number of team members upgrade PostgreSQL quietly (e.g. from 9.6.11 to 9.6.13), and this broke their GDK quietly because Procfile was pointing to the wrong path.

For example, instead of linking /usr/local/Cellar/postgresql@10/10.6_1/bin/postgres, we can just link /usr/local/opt/postgresql@10/bin/postgres.

It looks like this value is calculated via:

ruby support/pg_bindir

Perhaps we should try using which psql first?