formula_support: don't fail without CLT/Xcode

`Formula#keg_only?` could fail if it was invoked on a formula with a
`:provided_until_xcode43` or `:provided_until_xcode5` reason given to
`keg_only`, if neither the Command Line Tools nor Xcode was installed.
Check whether Xcode is installed before querying the Xcode version.

Closes #317.

Library/Homebrew/formula_support.rb

@@ -18,9 +18,9 @@ class KegOnlyReason
     when :provided_pre_el_capitan
       MacOS.version < :el_capitan
     when :provided_until_xcode43
-      MacOS::Xcode.version < "4.3"
+      MacOS::Xcode.installed? && MacOS::Xcode.version < "4.3"
     when :provided_until_xcode5
-      MacOS::Xcode.version < "5.0"
+      MacOS::Xcode.installed? && MacOS::Xcode.version < "5.0"
     else
       true
     end