Incompatibility of 1.2.* where symfony/contracts also exists

[irinikp]

If symfony/contracts is also used in the project, there versions 1.2.* have an issue:

TypeError : Argument 2 passed to Symfony\Component\EventDispatcher\EventDispatcher::dispatch() must be an instance of Symfony\Component\EventDispatcher\Event or null, instance of Finite\Event\StateMachineEvent given

This seems to occur when someone has both the Symfony\Contracts\EventDispatcher\Event and Symfony\Component\EventDispatcher\Event installed. This check in StateMachineEvent checks whether Symfony\Contracts\EventDispatcher\Event exists, and aliases it to Symfony\Component\EventDispatcher\Event if not.

[irinikp]

Submitted PR to resolve this issue #158