[3.0] Cleanup

[driesvints]

This PR does a couple of things:

• Bumps the minimum requirement for PHP to 7.1.3 (same as for our other repos & framework(
• Bumps the Laravel components to the latest supported version (5.7)
• Tests on 7.3 and removes tests for 7.0
• Refactors the way env variables are handled in tests (same as for Cashier)
• Cleans up the test suite and internal code, docblocks, etc

If you'd like me to send this in without the cleanup feel free to say so. But I think it's good if this cleanup could be merged. Just let me know if you spot a couple of things which you dislike.

This is for a new major version so there are a few small breaking changes to method signatures. Here are some:

• hasAddon renamed to hasAddOn to be conform with the rest of the AddOn naming
• Remove the $storagePath param from the downloadInvoice method like on Cashier

At the moment tests for PHP 7.3 are still failing because of a bug in the Braintree SDK. Waiting for a bugfix from their side before I bump the minimum PHP version: https://github.com/laravel/cashier-braintree/issues/61

Bumped the minimum Braintree version for the crucial bugfix from the above issue ^