Switch from C asserts to Objective-C asserts

via the @NSHipster [article on Assertions](http://nshipster.com/nsassertionhandler/)

> The second is the difference between C and Objective-C assertions: NSAssert should only be used in an Objective-C context (i.e. method implementations), whereas NSCAssert should only be used in a C context (i.e. functions).