RulerNode should handle dynamic layout

[marlitas]

While working on the Sound Waves prototype (phetsims/sound-waves#23) we discovered that the unit label runs off the end of the ruler with stringTest=dynamic.

While looking through the code for RulerNode we determined that a fix for this was not within the scope of the Sound Waves prototype, however it should be addressed when Sound Waves is picked up again for redesign. It may be worth going through other sims that use the RulerNode to double check if they have this same problem as well.