Support Laravel 6 and 7

[carcinocron]

• edited composer.json to allow versions of laravel 6+.
• remove array_* to not force users into requiring that optional package.
• added getKey function for laravel 7.

[carcinocron]

If this doesn't get merged, add to your composer.json:

    "repositories": [
        {
            "type": "vcs",
            "url": "http://github.com/InstanceOfMichael/laravel-aead"
        }
    ]

then run

composer require hernandev/laravel-aead:dev-master#7c34aa94509ff3390e28dfdafb1042aee058eeb3