[Fix] Solved a problem caused by 'where.not' when using the globalize gem

I recently found out that using `friendly_id` + `globalize` will break when the app is running with postgres as database.

In particular, calling `.where.not( ... )` on a scope will append `AND (NOT ('--- id: '))` to queries, thus breaking the query on the database.

This fix converts the `.where.not()` in a generic `where()` clause through `arel` syntax.
As this is only used once, do we need the variable assignment? Could we just call `self.class.arel_table` below?