Make ClassDescendants behave deterministically (#220)

Sort descendants by class name.  This ensures that generators are run in
a deterministic order.

(With this, plus changes to `xcodeproj`, plus manually calling
`predictabilize_uuids` in my Cakefile, I can get the same
project.pbxproj output across multiple runs.)

lib/xcake/core_ext/class.rb

@@ -7,7 +7,7 @@ module ClassDescendants
       # Returns all descendants of a class
       #
       def descendants
-        ObjectSpace.each_object(singleton_class).select { |klass| klass < self }
+        ObjectSpace.each_object(singleton_class).select { |klass| klass < self }.sort_by(&:to_s)
       end
     end
   end