Suppressing error message when CLTools not installed

On Mavericks you can install full Xcode and Boxen, but then you get this line every time you run `boxen` on the command-line:

    No receipt for 'com.apple.pkg.CLTools_Executables' found at '/'.

This minor bugfix suppresses the message.
Merge pull request #607 from walling/master

Suppressing error message when CLTools not installed