Fix ArcanistJSHintLinterTestCase::testLinter for recent JSHint

Summary:
Recent JSHint improves the warning and attributes it to the equals sign
rather than the end of the expression (changed in 897e0359ce19, first
released in 2.11.0-rc1).

Test Plan: Ran arc unit with JSHint 2.12.0

Reviewers: epriestley, joshuaspence, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21503