Don't use Homebrew for rbenv

* Use the same Git/GitHub approach for Mac as we do for Linux.
* This makes upgrading easier via `git pull`.
* Add idempotency `if` guards.
* Use bash-preferred bracket-bracket style.

http://robots.thoughtbot.com/the-unix-shells-humble-if
Fixes rbenv component path in the manifest files

Commit bc5a7c6 changed how rbenv is installed
("Use the same Git/GitHub approach for Mac as we do for Linux."),
but was missing the changes in the manifest files.

This commit fixes the corresponding manifest files.
Install Postgres via Hombrew bottle

Upstream maintainers put thought into what is
and what isn't sensible to include by default.
Those decisions establish a de facto standard
that is broader in scope than Laptop:
Homebrew users, library users, etc.

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:

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

Using options forces an install from source rather than a bottle,
which is 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.