Fix for Issue #29

This removes the type hint for `Collection::contains` to match Laravel 4.2's [current signature for this method](https://github.com/illuminate/support/blob/8d389be3efdc62cb56d35997930299e1a7a7245a/Collection.php#L81) with no type hint. Unit tests still pass. This is the only method in the `Collection` class that overrides anything from the Illuminate Collection.