[5.4] Add whereNotIn() to Collection

[guidocella]

This PR adds the whereNotIn and whereNotInStrict methods to the Collection class.

Turns

$collection->reject(function ($item) {
    return in_array($item->key, ['foo', 'bar']);
});

into

$collection->whereNotIn('key', ['foo', 'bar']);

[taylorotwell]

This changes the method signature of whereIn so it is a breaking change.