Laravel 6 Support

[jbeales]

Supports Laravel 6, (with Monolog 2).

Summary

• Changed the signature of MailableHandler::send() to match the new signature of Monolog\Handler\MailHandler::send() that it's inheriting from.
• Changed the signature of the MailableHandler::__construct() to force int and bool types for $level and $bool, respectively, (they're passed to Monolog\Handler\AbstractHandler::__construct() which requires int & bool).
• Added default debug level and bubble config in MailableLogger::__invoke(). If $level and $bubble are not set, and the correct types, instantiating a new MailableHandler, (or any handler), fails silently.

Notes:

• I haven't bumped the version or anything. I believe this will still work with Laravel 5, but haven't tested.
• If someone puts an invalid value for level or bubble in their config file the logger will fail silently.