Add Laravel 6 support

The major change in this release is the addition of support for Laravel 6.
Additional changes:

• Added the ability to map individual event classes to individual SNS topic ARNs
• Added the ability to disable automatic response to subscription events from SNS topics
• Added a check to ensure that event classes mapped to ARNs are dispatchable before attempting to dispatch them
• Updated documentation to reflect new features

Corrections to documentation

Addressed some minor formatting issues in the README and added information about the route needed to subscribe the SNS topic.

First production release

This initial release supports subscribing to SNS topics in Laravel 8.