Add integer parameter to Divisibility Intuition

Summary: Numeric keyboards on mobile.

Test Plan:
Visited exercise in Mobile Safari/iPad. Verified numeric keyboard appeared when
touching an input box.  Visited exercise in Chrome/Mac OS X. Verified exercise
still works.

Reviewers: tom

Reviewed By: tom

CC: eater

Differential Revision: http://phabricator.khanacademy.org/D1020