Use `tree` as customization example

Laptop should be responsible for setting up
the compilers,
system package manager,
programming language version managers & installers,
large system dependencies such as Postgres,
and any critical config for those tools
which is often overlooked but can cause bugs and headaches.

Other useful command-line tools such as `watch` and `tree`
are better installed in individuals' `~/.laptop.local` files.

Add `tree` as a good example for that file.

Try to make `~/.laptop.local` explanation clearer
and more prominent by placing it in a heading.

Remove Suspenders as an example.
Right before using Suspenders,
it's probably best to re-install it to get the latest.
Be more specific in printed output

* Say whether we are Installing, Upgrading, or Skipping.
* Changes the behavior of `brew_install_or_upgrade`
  but simplifies and shortens the rest of the script.
Use Homebrew for Git

We previously relied on the version of Git
that comes with Xcode.
The Homebrew version is easier to upgrade
when new versions of Git are released,
such as yesterday's security patch:

https://github.com/blog/1938-git-client-vulnerability-announced
Use rbenv shell instead of rbenv local

`rbenv local` creates a `.ruby_version` file
in the current directory.
Install, upgrade, or skip Bundler
Add gem_install_or_update function

* Use it for Bundler.
* Document it for others' use in their ~/laptop.local.
* Delete out of date part of README.