Prefer Xcode 6.1 on 10.10

More Yosemite changes. Within two weeks or so, Xcode should be made
available on the App Store, at which point 10.9 will need to go from
“5.1.1” to “6.0” but whilst Yosemite is in Beta *everyone* should be
using the Xcode Beta builds according to Apple, so Yosemite should be
on 6.1 for the foreseeable, even when Apple releases Xcode 6.0 to 10.9
& below. 6.1 is still using the same Clang version number at this point.

Closes #32201.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>