Unbreak TransitionScrollStrategy

This was broken by the change that added fixedTime and frame
properties to TouchScrollStrategy; the observers for those
properties assumed that scrollMath always existed, but it doesn't
for TransitionScrollStrategy, so we'll guard those calls.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #531 from enyojs/fix-transition-scroll-strategy

Unbreak TransitionScrollStrategy