Fix build problem on Xcode < 7.3 #30 #31

src/ios/RNFetchBlobFS.m

                             else
                                 asciiStr = [asciiStr stringByAppendingFormat:@"%d", val];
                         }
+                        free(bytePtr);
                     }
                     asciiStr = [asciiStr stringByAppendingString:@"]"];
                     [self.bridge.eventDispatcher

src/ios/RNFetchBlobNetwork.h

 @property (nonatomic) int expectedBytes;
 @property (nonatomic) int receivedBytes;
 @property (nullable, nonatomic) NSMutableData * respData;
-@property (nullable, nonatomic) RCTResponseSenderBlock callback;
+@property (strong, nonatomic) RCTResponseSenderBlock callback;
 @property (nullable, nonatomic) RCTBridge * bridge;
 @property (nullable, nonatomic) NSDictionary * options;
 @property (nullable, nonatomic) RNFetchBlobFS * fileStream;
-@property (nullable, nonatomic) CompletionHander fileTaskCompletionHandler;
-@property (nullable, nonatomic) DataTaskCompletionHander dataTaskCompletionHandler;
+@property (strong, nonatomic) CompletionHander fileTaskCompletionHandler;
+@property (strong, nonatomic) DataTaskCompletionHander dataTaskCompletionHandler;
 @property (nullable, nonatomic) NSError * error;