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:

Homebrew/homebrew@afa4549

I confirmed that it works on a clean 10.9 install.

We also should not instruct the user to run `sudo xcodebuild -license` at all.

Here's why:

The `xcodebuild program` isn't included in OS X Mavericks. When you run
`xcodebuild`, you're actually finding one of 83 shims found in `/usr/bin` that
are included in Mavericks. These shims are an important part of how the
`xcode-select` mechanism works. When the command line developer tools have not
been installed, invoking any of these shims won't do anything other than prompt
you to install the command line developer tools:

    $ xcodebuild -license
    xcode-select: note: no developer tools were found at
    '/Applications/Xcode.app', requesting install.
    Choose an option in the dialog to download the command line developer tools.

(And, the GUI install dialog is presented.)

Of course, since the real `xcodebuild` program isn't actually available, it
doesn't make sense to try and use it to accept the Xcode License Agreement.

Instead, go straight to installing with `xcode-select --install` (xcode-select
comes with OS X Mavericks and is not a shim):

    $ xcode-select --install
    xcode-select: note: install requested for command line developer tools

And, the exact same GUI install dialog is presented (prompting the user to
either "Get Xcode" from the App Store or immediately "Install" the command line
developer tools).

For more about `xcode-select` and it's shims, see `man xcode-select`.

This also removes redundant Vagrant setup instructions and capitalize Vagrant.