[4.0] Updated to PHPUnit 6 and Mockery 1

[carusogabriel]

Updated phpunit/phpunit to ~6.0 and mockery/mockery to ~1.0.

Dropped support to PHP 5.6, as PHPUnit 6.0 requires PHP 7.0.

PS: I've used this commit from laravel/framework to prevent failed tests.

[browner12]

I would update the illuminate dependencies as well to 5.5.

but also, IMHO, don't think justifies a new major version.

[carusogabriel]

@browner12 If @taylorotwell merge it, I'll update the illuminate dependencies as well.