Switch to afterResolving when registering service provider

[TheoGibbons]

This helps fix the conflict with therobfonz/laravel-mandrill-driver on Laravel 7.
They will also need to fix their ServiceProvider before the conflict is fully resolved.
#84

[juukie]

Thanks @TheoGibbons. Hopefully @fedeisas will be able to merge and tag soon.

[lbeauvisage]

Thank you so much @TheoGibbons , it also fix the problem form vemcogroup/laravel-sparkpost-driver and probably s-ichikawa/laravel-sendgrid-driver

With Laravel 7 it cause the error

InvalidArgumentException
Unsupported mail transport [sparkpost].