Fix more memory bug.

1、XHMessageTextView the - (void)insertText:(NSString *)text  method
memory leak, delete the method fix it bug.
2、UIImage+XHRounded CGImageRef imageMasked =
CGBitmapContextCreateImage(context); the row must be release
CGImageReg, fix it bug for CGImageRelease(imageMasked);

fix https://github.com/xhzengAIB/MessageDisplayKit/issues/16