Switch to sha256

From https://www.comodo.com/e-commerce/SHA-2-transition-next-steps.php

quoting Microsoft:
"For code signing certificates, Windows will stop accepting SHA1 signed code and SHA1 certificates that are time stamped after 1 January 2016. SHA1 signed code time stamped by an RFC 3161 Time Stamp Authority before 1 January 2016 will be accepted until such time when Microsoft decides SHA1 is vulnerable to pre-image attack."

I haven't thoroughly tested the patch