Split up unreadable conditional
Update comment about CLT

This was worded in a confusing way, and made an assumption about future
OS X versions that we don't know to be true.

Also, it's probably wrong to state that the CLT location is dependent on
the OS X version; surely there will be a newer CLT for 10.8 that gets
installed in the same location as on 10.9. But we don't know any better
yet.
Pull another unwieldy condition into a method
Existence of directory is implied by existence of executables
Extract bundle_path method from Xcode#prefix
Eliminate nested conditional