Release 2.0

• PHP >= 7.1 only release. Thanks @TheLevti for assisting.
• This version adds type hints to arguments and return types to methods. If you have your own implementations of the Mutex class, you might need to update it slightly.
• The Memcache Mutex has been removed. You should use the Memcached mutex instead.
• The Predis and PHPRedis mutexes have been updated to better follow the distlock from the Redis documentation.
• A bug has been fixed in the Predis mutex where failing to release a lock could cause a fatal error if sentinel replication was used.
• The return value of the callable passed to then() is now returned (#23). Thanks @TheLevti.
• Various testing and CI improvements.