Allow eager load constraints to still apply for pivot relations.

[jasonlewis]

Currently the following won't be constrained:

Plan::with([
    'items.pivot.unit' => function ($query) {
        $query->where('enabled', true);
    }
])->get()

The function is filtered out. This commit preserves the constraining function passed in. Note that I've used ARRAY_FILTER_USE_KEY which was made available in PHP 5.6 so I've bumped the composer.json PHP version as well. Granted, Laravel 5.2 is the oldest version of Laravel that would still work on PHP prior to 5.6.

Cheers.

[ajcastro]

Nice. Thanks for this @jasonlewis !