Update package to work with stable version of ChatKit SDK

[IMPMAC]

Hi, Thanks for you package.

Now that ChatKit is out of beta, they will turn off support for older version of the API, I have attempted to update the SDK but I get this error.

`Your requirements could not be resolved to an installable set of packages.

Problem 1
- chess/laravel-chatkit-api v0.2.1 requires pusher/pusher-chatkit-server ^0.5.0 -> satisfiable by pusher/pusher-chatkit-server[0.5.0, 0.5.1, 0.5.2, 0.5.3, 0.5.4, 0.5.5, 0.5.6, 0.5.7, 0.5.8, 0.5.9] but these conflict with your requirements or minimum-stability.
- chess/laravel-chatkit-api v0.2.1 requires pusher/pusher-chatkit-server ^0.5.0 -> satisfiable by pusher/pusher-chatkit-server[0.5.0, 0.5.1, 0.5.2, 0.5.3, 0.5.4, 0.5.5, 0.5.6, 0.5.7, 0.5.8, 0.5.9] but these conflict with your requirements or minimum-stability.
- Installation request for chess/laravel-chatkit-api ^0.2.1 -> satisfiable by chess/laravel-chatkit-api[v0.2.1].
`
Could you help on this matter?
Thanks.