Resolve "Gracefully handle PostgreSQL minor updates in Procfile"

robust_pg_bin_dir_discover aims to locate a PostgreSQL bin directory for 9.6 that won't change with patch upgrades.

If robust_pg_bin_dir_discover does not return anything then original_pg_bin_dir_discover is executed.

robust_pg_bin_dir_discover

$ support/pg_bindir
/usr/local/opt/postgresql@9.6/bin

original_pg_bin_dir_discover

$ mv /usr/local/opt/postgresql@9.6 /usr/local/opt/postgresql@9.6.temp

$ support/pg_bindir
/usr/local/Cellar/postgresql@9.6/9.6.13/bin

Closes #498 (closed)