I did some research and my best guess as to the cause of this error message is
that the executable is signed using SHA1 after the date SHA1 was supposed to
have been deprecated (some time in December 2015).
The file downloaded without complaint in Chrome. I checked the file with
McAfee and it didn't find anything. File | Properties | Digital Signatures
from Windows File Explorer shows that the signature is actually valid (if you
discount that SHA1 was used).
I think it's safe to ignore the error message.