Add missing underscore argument to recalculateAnimationLength

Hi Jeremy,

I found a small bug in the recalculateAnimationLength function. It's missing a _ argument.
This bug was not caught by unit tests because underscore is loaded globally via a script tag in all qunit tests. I found it because I load rekapi with requirejs in my app.

I think it may be safer to convert all qunit tests to load rekapi and its dependencies via requirejs if we don't want to miss these kind of bugs in the future.