Fix IOS fs.df overflow problem #276

ios/RNFetchBlobFS.m

         totalSpace = [fileSystemSizeInBytes unsignedLongLongValue];
         totalFreeSpace = [freeFileSystemSizeInBytes unsignedLongLongValue];
         callback(@[[NSNull null], @{
-                  @"free" : [NSString stringWithFormat:@"%d", totalFreeSpace],
-                  @"total" : [NSString stringWithFormat:@"%d", totalSpace]
+                  @"free" : [NSString stringWithFormat:@"%ld", totalFreeSpace],
+                  @"total" : [NSString stringWithFormat:@"%ld", totalSpace]
                 }]);
     } else {
         callback(@[@"failed to get storage usage."]);