emacs: enable parallel HEAD builds

Formula/emacs.rb

@@ -51,11 +51,6 @@ class Emacs < Formula
   end
 
   def install
-    # HEAD builds blow up when built in parallel as of April 20 2012
-    # FIXME is this still necessary? It's been more than two years, surely any
-    # race conditions would have made it into release by now.
-    ENV.deparallelize unless build.stable?
-
     args = ["--prefix=#{prefix}",
             "--enable-locallisppath=#{HOMEBREW_PREFIX}/share/emacs/site-lisp",
             "--infodir=#{info}/emacs"]