Fix iOS 6 crashing with VoiceOver on

Check at runtime if this is iOS 7.  (This was crashing on iOS 6 devices built against the iOS 7 SDK.)

Note my gross hack here:

```
- (BOOL)shouldSupportAccessibilityProtocol {
    return ([NSURLSession class] != nil);
}
```

I tried testing for `NSTextStorage`, etc., but those were actually returned on iOS 6.  (I guess they were there, but private before.)

I'd like some commentary on a better run-time test, and also if anyone thinks this value should be cached.

Fixes https://github.com/mattt/TTTAttributedLabel/issues/396 and hopefully https://github.com/mattt/TTTAttributedLabel/issues/397 (although I couldn't reproduce the latter.)

CC @mattt @jhersh @sticksen @ksm