Simplify usage by supporting new default loop

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This changeset simplifies usage by supporting the new default loop.

// old (still supported)
$factory = new React\Datagram\Factory($loop);

// new (using default loop)
$factory = new React\Datagram\Factory();

Builds on top of reactphp/event-loop#226, reactphp/event-loop#229 and reactphp/event-loop#232
Refs reactphp/stream#159

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The first commit updates this to the currently unreleased EventLoop component to show how the new default loop can be used in this component (the build should be green). The second commit updates this to the release that has yet to be tagged. This is expected to fail at the moment and should be green once the release is tagged and the build is restarted.

This is ready for review. I will restart the build and merge once reviewed and the EventLoop component is tagged.

[clue]

Restarted build and everyhing's green now that reactphp/event-loop v1.2.0 has been released