Avoid deprecation messages on PHPUnit 8

[mnabialek]

assertContains and assertNotContains are deprecated in PHPUnit 8, so we need to use assertTrue and assertFalse together with Str::contains if we want to also support PHPUnit < 7.5

[driesvints]

Heya, thanks for sending this in. I've, however, replaced these with the proper PHPUnit assertions instead of using our own Str helper. Here's the commit: d48f990

[mnabialek]

@driesvints But it won't work when someone is using PHPUnit 7, those assertions were introduced in PHPUnit 7,5 I believe and in composer.json we allow any PHPUnit 7 version

[driesvints]

@mnabialek good catch 👍

Wasn't aware that they were only added then.