Collection: allow $preserve_keys option on reverse()

Is there a reason the `Collection`'s reverse method doesn't have a `$preserve_keys` option (see [PHP Docs for array_reverse](http://php.net/manual/en/function.array-reverse.php))? Is it simply a PHP version issue?

I ended up having to use `collect(array_reverse($points->all(), true))` instead of what I hoped would just be `$points->reverse(true)`.
