Add a linter rule for abstract methods within an interface

Summary:
interfaces cannot contain abstract methods. This construct will cause a PHP fatal error:

Access type for interface method SomeInterface::someMethod() must be omitted

Test Plan: Added test cases.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14562