Load sha256, so we can interact with the new PAymill certificate.

Background: After heartbleed, paymill propably replaced its SSL certificate. It seems it also changed
something else, like the alorithm. After 8.4.2014 we received errors when talking to paymill:

Get https://api.paymill.de/: x509: certificate signed by unknown authority (possibly because of "x509: cannot verify signature: algorithm unimplemented" while trying to verify candidate authority certificate "COMODO RSA Certification Authority")

This change loads the sha512 package, which seems to fix this. Magic.