Revert "Use erlang:'<'/2 directly for raw collation"

This commit adds the old comparison function.
It's based on the revert from
http://review.couchbase.org/10505

The revert got lost during the index merger refactoring.

Change-Id: I36ad6496c033be881b7a7bbf7c6144b9e2a23f56
Reviewed-on: http://review.couchbase.org/11305
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>

src/couchdb/couch_view_merger.erl

@@ -205,7 +205,7 @@ view_less_fun(Collation, Dir, ViewType) ->
             fun({KeyA, _}, {KeyB, _}) -> couch_view:less_json(KeyA, KeyB) end
         end;
     <<"raw">> ->
-        fun erlang:'<'/2
+        fun(A, B) -> A < B end
     end,
     case Dir of
     fwd ->