Add hhvm.

Signed-off-by: crynobone <crynobone@gmail.com>
Release v2.1.0

Signed-off-by: crynobone <crynobone@gmail.com>
Implement Orchestra\Notifier\NotifiableInterface.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge branch '2.1'

Signed-off-by: crynobone <crynobone@gmail.com>

Conflicts:
	composer.json