Fix images not reloading after memory warning

Reviewed By: mmmulani

Differential Revision: D3816779

fbshipit-source-id: edccb41a38fff3b2c143ffb3cbe0a7001ee16331

Libraries/Image/RCTImageView.m

@@ -214,6 +214,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   [self cancelImageLoad];
   [self.layer removeAnimationForKey:@"contents"];
   self.image = nil;
+  _imageSource = nil;
 }
 
 - (void)clearImageIfDetached