Fix dice_probability JS error in local mode

When loading the exercise in local mode, the <script> block would immediately
run before `KhanUtil` was defined, resulting in the screen-grabber-thing
complaining about `ReferenceError: Can't find variable: KhanUtil`. Instead this
defines the `selectMatching()` function on the `currentGraph.graph` object
which is nicely scoped to graphies.

Test Plan:
Load http://exercises.ka.local/exercises/dice_probability.html and don't see a JS error in the console.
Take hints and see things work

Auditors: emily