Changed use of $ to jQuery.

Fixes #1288

In the referenced project, using Browserify, jQuery will not add `$` or `jQuery` if the `window` object doesn't exist.  The scope plugin looks for `$` on the window object but wouldn't find it and would therefore not attach itself.  Browserify-shim can be used to create the global jQuery, but CanJS was still looking for `$`.

Changed the check from `$` to `jQuery`.