Update prerequisites in README

Homebrew's install script checks whether the command line tools are installed,
and, only if necessary, will run `xcode-select --install`.

https://github.com/Homebrew/homebrew/blob/go/install#L162

The test they are using depends in part on a heuristic:

https://github.com/Homebrew/homebrew/commit/afa45493af041e3357d9cff64dea085e88eed251

@gohanlon confirmed that it works on a clean 10.9 install.

Also remove redundant Vagrant setup instructions and capitalize Vagrant.