Fixed compiler warning, for using "NSURLCacheStorageNotAllowed"

Replaced reference to `NSURLCacheStorageNotAllowed` with `NSURLRequestReloadIgnoringLocalCacheData`, which is of the type the method expects, and has the same intended effect.

This StackOverflow answer also indicates this as a proper solution: http://stackoverflow.com/a/20739049/105717

(addressing issue #57)