Fix wrong stack processing order

[dionysiosarvanitis]

Calling first on a collection triggers Arr::first which changes the
order of key,value when executing the callback.

See Illuminate\Support\Arr : L147

[BenConstable]

Great spot on this, thanks very much.

The order of parameters to Arr::first() was changed in Laravel 5.3 (see "Arrays" section here) As I want this package to support 5.1 and up, I've removed the use of the callback to first() in favour of filter(), as this supports all versions of Laravel from 5.1 and up.

You can see the change in e695f5f.

[BenConstable]

Also, I've updated the TravisCI config to test individually against each "major" Laravel version, so these kind of problems should be picked up by the tests now.

[dionysiosarvanitis]

I imagined that something like that would have happen. Thanks for taking care of this.