Add jquery.ui.dialog (and dependencies)

Summary:
The chemistry exercises require jquery.ui.dialog, which wasn't
previously included. THis includes it, and updates the update_local script.

Test Plan:
Visit /exercises/ideal_stoichiometry.html and note that the problem
loads correctly, and that when you press "show periodic table", a periodic
table dialog opens.

Reviewers: alpert, csilvers

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2522