GitHubPayloadAnalyzer exception when payload isn't an object

Summary:
GitHubPayloadAnalyzer constructor requires a $payload parameter,
which is the deserialization of the JSON GitHub payload.

The constructor now checks it's indeed an object, or throws an
exception if not.

Test Plan: Fire a Phabricator payload to /gate/GitHub/projectWithoutSecret

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T695

Differential Revision: http://devcentral.nasqueron.org/D261