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