Silence 64 bit warning

When building for 64 bit devices, the following warning is triggered in `[CPAnimationStep description]`:

~~~
CPAnimationSequence/Component/private/CPAnimationStep.m:115:40: Format specifies type 'int' but the argument has type 'UIViewAnimationOptions' (aka 'enum UIViewAnimationOptions')
~~~

This casts the `NSUInteger` causing that issue to an `unsigned long`, silencing the warning.