Cache MacOS.version comparison results

MacOS.version#<=> is called many, many times during formula loading with
the same half dozen or so arguments. A typical call to this method
involves:

 * a hash lookup to convert a symbol argument to a string
 * creation of a throw-away Version object wrapping the argument
 * the actual version comparison, which is not cheap

This makes it a prime candidate to be memoized.

Library/Homebrew/os/mac/version.rb

@@ -16,9 +16,16 @@ module OS
         new(SYMBOLS.fetch(sym))
       end
 
+      def initialize(*args)
+        super
+        @comparison_cache = {}
+      end
+
       def <=>(other)
-        v = SYMBOLS.fetch(other, other.to_s)
+        @comparison_cache.fetch(other) do
-        super(Version.new(v))
+          v = SYMBOLS.fetch(other, other.to_s)
+          @comparison_cache[other] = super(Version.new(v))
+        end
       end
 
       def to_sym