Stop exposing mktemp as a public method on formula objects

Library/Homebrew/extend/fileutils.rb

@@ -23,6 +23,7 @@ module FileUtils extend self
     cd prevd if prevd
     ignore_interrupts{ rm_r tempd } if tempd
   end
+  module_function :mktemp
 
   # A version of mkdir that also changes to that folder in a block.
   alias mkdir_old mkdir

Library/Homebrew/formula_versions.rb

@@ -52,7 +52,7 @@ class FormulaVersions
   end
 
   def formula_at_revision rev, &block
-    f.mktemp do
+    FileUtils.mktemp(f.name) do
       path = Pathname.pwd.join("#{f.name}.rb")
       path.write file_contents_at_revision(rev)