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* Delete out of date part of README.