Use Postgres bin path

Merged DJ Mountney requested to merge twk3/gitlab-development-kit:postgres_fullpath into master

Postgres on Ubuntu doesn't include the initdb and postgres commands on the PATH, but are still available from the postgres bin directory.

Updated the Makefile to use pg_config to find the postgres bin directory, and use it for the initdb command, and for the postgres command in the procfile

Works towards completing issue #7 (closed)