xcodeversion fact always tries to install xcode

Whenever I called "xcodebuild" on Mavericks it would prompt me to install the command line utils.

Also, I think the `return "" if ! File.exists?('/usr/bin/xcodebuild')` causes an error in facter if it doesn't exist (all explicit returns were causing errors for me in facter).

This patch creates a more specific fact for Mavericks (which, since it matches two confines it takes a higher precedence) and just checks for the dirs that are normally created by xcode and xcode cli utils.