feat: update DiscordPHP to the latest version

[owenvoke]

I've also removed the Travis CI config as there are no tests so it fails with the latest version of PHPUnit. And also removed the PHP-CS-Fixer config as it is not being used as there is StyleCI. It also looks like the sllh/php-cs-fixer-styleci-bridge package is no longer maintained.

As there are no tests, I'm going to try and manually test this in a Botman application, which is why this is marked as draft.

[ping-localhost]

Did you have any luck getting this working @owenvoke?

[owenvoke]

@ping-localhost, unfortunately I can't even remember 🤦🏻 I think it was mostly working, but I've ended up using DiscordPHP's Slash Command package instead for bots recently.