Unreference wrapped function in _.once

Assuming we'll never run the wrapped function again on _.once(), we can
assign null to the `func` variable, so function (and all its inherited
scopes) may be collected by GC if needed.

underscore.js

@@ -623,7 +623,9 @@
     return function() {
       if (ran) return memo;
       ran = true;
-      return memo = func.apply(this, arguments);
+      memo = func.apply(this, arguments);
+      func = null;
+      return memo;
     };
   };