Include extension name for GDSetupCheck.

Summary: Help for GD SetupChcek was missing the "how to install extension" content.

Test Plan: Uninstalled gd, validated extension installation instructions were present in Setup Issue.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D18922