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* Delete out of date part of README.
Simplify return value negation
Remove glob within test command

- Globs within [[ is a bashism
Use grep -F for speedup on non-regex
Replace [[ bashism with POSIX [
Replace source bashism with POSIX .
Use $HOME consistently over ~
Replace (( bashism with POSIX [
Remove process substitution bashism
Use /bin/sh shebang
Replace printf | grep with a case