Declare `Raphael` as a global for JSHint

Summary: Let JSHint know that `Raphael` is a global that can be used anywhere. This is not technically correct, but it silences a few JSHint warnings. See http://jshint.com/docs/ for more information.

Test Plan: `arc lint -- webroot/rsrc/js/application/maniphest/behavior-line-chart.js`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11069