Merge if you like math!

GMP is no longer a dependency. If GMP does not exist, BC Math is used instead.
You can probably refactor this to

```php
if (!extension_loaded('bcmath') || !extension_loaded('gmp')) {
    $this->markTestSkipped('BCMath extension not loaded, GMP extension not loaded.');
}
```

Although you might just want to do the BCMath extension.