Replace XCode CLT hack with xcode-select

As of OS X 10.9, the `xcode-select` command now comes standard with
Macs, allowing you to install XCode's command line tools.

Since I have no intention of supporting older versions of OS X when the
alternative is so much easier and less prone to error, switch to using
`xcode-select`.
Update Homebrew install URL

The URL was recently changed, breaking the install script. Update it.