drop firefox 3.6 support

firefox 5 is out, and even with this line I couldn't get 4chan x working
in fx 3.6

4chan_x.user.js

@@ -479,9 +479,6 @@
       return _results;
     };
   }
-  if (!d.head) {
-    d.head = $('head', d);
-  }
   $$ = function(selector, root) {
     if (root == null) {
       root = d.body;

script.coffee

@@ -330,9 +330,6 @@ if not Object.keys
   Object.keys = (o) ->
     key for key in o
 
-# upgrade your firefox god damn it!
-if not d.head then d.head = $('head', d)
-
 $$ = (selector, root=d.body) ->
   Array::slice.call root.querySelectorAll selector