add default implementation for upsertQuery

Some dialects developers might need to be reminded they need to
implement the upsertQuery.

lib/dialects/abstract/query-generator.js

@@ -329,6 +329,13 @@ module.exports = (function() {
     },
 
     /*
+      Returns an upsert query.
+    */
+    upsertQuery: function (tableName, insertValues, updateValues, where, rawAttributes, options) {
+      throwMethodUndefined('upsertQuery');
+    },
+
+    /*
       Returns a deletion query.
       Parameters:
         - tableName -> Name of the table