Fix PostgreSQL installation on Homebrew

Quentin Pradetrequested to merge
quentinp/gitlab-development-kit:homebrew-install-pg into master

PostgreSQL on Homebrew is "keg-only", which means the binaries are not in PATH after installing. Homebrew suggests doing this manually if needed. Doing it fixes the installation of the pg gem and ensures postgres_bin_dir is correctly defined in the Makefile.

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