Install Postgres via Hombrew bottle

Upstream maintainers put thought into what is and isn't sensible to include by
default, and those decisions establish a de facto standard that's broader in
scope than Laptop (e.g. Homebrew users, library users).

The further we stray from the defaults the more likely we are to run into
problems, and those problems will be harder. The most widely experienced, most
urgent and most rapidly fixed problems will be problems with installing using
formula defaults. Problems from using non-defaults are relatively lonely.

Also, Homebrew bottles are awesome, but using options forces an install from
source rather than a bottle — it's much, much slower:

> Options were passed to the install command i.e. `brew install $FORMULA` will
> use a bottled version of `$FORMULA`, but `brew install $FORMULA --enable-bar`
> will trigger a source build.

https://github.com/Homebrew/homebrew/wiki/FAQ#why-do-you-compile-everything