Fix the ArcanistJSHintLinter::getVersion function.

Summary: As identified by @epriestley in D9060, jshint --version emits version information on stderr, not stdout.

Test Plan: Ran jshint --version locally and verified that the version information is output to stderr.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T4954

Differential Revision: https://secure.phabricator.com/D9173