Fix invalid "USING" order in Postgres query generator

According to documentation (http://www.postgresql.org/docs/9.3/static/sql-createindex.html) USING parameter must follow ON, not vise-versa. You can reproduce this bug through migrations by adding index with custom indexType:

```javascript
migration.addIndex('table', [ 'somefield' ], {
	indexType: 'hash'
});
```