fixed warnings for lack of precision when building promise for 64-bit.

The collection returns an NSUInteger for -count.

From apple's Header:
````
#if __LP64__ || (TARGET_OS_EMBEDDED && !TARGET_OS_IPHONE) || TARGET_OS_WIN32 || NS_BUILD_32_LIKE_64
typedef long NSInteger;
typedef unsigned long NSUInteger;
#else
typedef int NSInteger;
typedef unsigned int NSUInteger;
#endif
`````

As such casting this message to int on a 64-bit build produces a warning about a lack of precision in the receiving type. Fixed by using the NSUInteger type.