Don't use Pathname#/ in performance-critical code

This method (well, really, #join) is *twice* as slow as simple
concatenation, and shouldn't really be used at all in non-Formula code.

Library/Homebrew/formula.rb

@@ -143,7 +143,7 @@ class Formula
   end
 
   def prefix
-    HOMEBREW_CELLAR/name/version
+    HOMEBREW_CELLAR+name+version
   end
   def rack; prefix.parent end
 

Library/Homebrew/formula_pin.rb

 require 'fileutils'
 
 class FormulaPin
-  HOMEBREW_PINNED = HOMEBREW_LIBRARY/'PinnedKegs'
+  HOMEBREW_PINNED = HOMEBREW_LIBRARY+'PinnedKegs'
 
   def initialize(formula)
     @formula = formula
     @name = formula.name
     HOMEBREW_PINNED.mkdir unless HOMEBREW_PINNED.exist?
-    @path = HOMEBREW_PINNED/@name
+    @path = HOMEBREW_PINNED+@name
   end
 
   def pin_at(version)