Switch to launchctl for Postgres

`brew services` is no longer supported:

https://github.com/Homebrew/homebrew/commit/c0b99c031fec0a2096b85a458d7b42e3627560d8

* Fixes https://github.com/thoughtbot/laptop/issues/93
* Fixes https://github.com/thoughtbot/laptop/issues/243
* Idempotently creates Postgres database cluster and starts (or restarts)
  Postgres.