Refuse to create new extension if legacy code is present on database

Closes #20

cartodb_version.sql.in

@@ -1,3 +1,5 @@
+DO $$ BEGIN IF EXISTS (SELECT * FROM pg_proc p, pg_namespace n WHERE p.proname = 'cdb_transformtowebmercator' AND p.pronamespace = n.oid AND n.nspname = 'public') THEN RAISE EXCEPTION 'Use CREATE EXTENSION cartodb FROM unpackaged'; END IF; END; $$ LANGUAGE 'plpgsql'; -- forbid duplicated extension
+
 CREATE OR REPLACE FUNCTION cartodb.CDB_version()
 RETURNS text AS $$
   SELECT '@@VERSION@@'::text;