Test a GitHub ping payload, events disabled

Summary:
Test purpose

This test fires the first GitHub event we received, a ping
with a zen aphorism. A blank report should be received,
with correct gate an door.

Services service provider change

To play nice with the Laravel testing recommandation to use
a Illuminate\Contracts\Events\Dispatcher mock for the events
dispatcher, we update the ServicesServiceProvider expectation
to use the interface contract, not the actual implementation.

New methods to TestCase

• disableEvents to mock the events dispatcher
• sendJsonPayload allows to send an object as JSON payload
• sendPayload allows to send a raw payload

Laravel offers a way to disable events from testing, but this
only expects the fire method, not a listen method.

Test Plan: phpunit --no-coverage

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D195