Update to iOS SDK 3.20, fix uses of /me/permissions

* Fix uses of /me/permissions that depend on pre v2.0 behavior.
* Update to iOS SDK version 3.20.
The code that checks the current permissions is repeated in a few places. Consider pulling this out into a private method. Something like:

    - (NSArray *) getCurrentPermissions:(NSArray *)returnedPermissions {
      NSMutableArray *currentPermissions = [@[] mutableCopy];
      for (NSDictionary *perm in returnedPermissions) {
          if ([perm[@"status"] isEqualToString:@"granted"]) {
                [currentPermissions addObject:perm[@"permission"]];
          }
      }
      return currentPermissions;
    }