Removed build warnings when using iOS 8.x or OS X 10.10 SDKs for NSDateComponents

Here's a potential solution to dealing with the build warnings that show up in RKISO8601DateFormatter.m when using the latest SDKs.

Please review.
I would be consistent with MAX vs MIN.

Also, it might be easier to just use the newer values in the code, and use #defines if they are not defined by the current headers.

For example,
```
#if __IPHONE_OS_VERSION_MAX_ALLOWED < 80000 || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_10
#define NSCalendarUnitYear NSYearCalendarUnit
#define NSCalendarIdentifierGregorian NSGregorianCalendar
..
#endif
```