Enable unit tests for `ArcanistJscsLinterTestCase`

Summary:
https://github.com/mdevils/node-jscs/issues/444 has been resolved in commit [[https://github.com/mdevils/node-jscs/commit/0de667bac2491ba04d930ff94e990965213ba36b | 0de667bac2491ba04d930ff94e990965213ba36b]], which means that `jscs` can be easily run on files with any extension.

NOTE: JSCS version 1.5.7 is required.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9789