[5.5] Add whereKeyNot to Eloquent Builder

[antonkomarev]

This PR introduces inverse method of the whereKey named whereKeyNot.

I've found it useful to write:

$query->whereKeyNot($id);

Instead of:

$query->where($this->getKeyName(), '!=', $id);

Implemented exactly as whereKey. It accepts single id, array of ids and collections.