Fix missing config values in webhook testing

[dennislindsey]

Resolves #37

[dennislindsey]

Tests are failing for PHP 5.6 and 7.0 because phpunit no longer supports them as they've both reached end-of-life.

[barryvdh]

Why did you add void return types? They are not supported in older PHP versions.

[dennislindsey]

I mistakenly ran the tests to confirm the change using my globally-installed version of phpunit rather than the one that ships with the project. The tests were failing because of the missing return types since phpunit no longer supports PHP versions 5.6 and 7.0. I removed the return types and re-ran the tests using the provided version of phpunit.