In removeTween(), check that _activeTweens contains tween to be removed.

Calling removeAllTweens() or stopTween() in an onComplete function will cause the
tween to be removed from the _activeTweens list.  When removeTween() is then called in the GoKitLite Update() function, that tween is no longer in the list and  _activeTweens.RemoveAt() will throw an exception.