fix wiki page spec

we were getting this a bunch when running karma specs:
Uncaught TypeError: Cannot read property 'isDirty' of undefined
	at .../public/javascripts/tinymce.editor_box.js:387

Change-Id: Iddcddc6fd84dbd0427262925db9077cc20e10b46
see: http://cl.ly/image/2J2r03171l2K
Reviewed-on: https://gerrit.instructure.com/42112
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>