Change SHA field labels to SHA1/SHA256 in Release file

These were previously called "SHA1Sum" and "SHA256Sum" which is not
correct according to
https://wiki.debian.org/RepositoryFormat#MD5Sum.2C_SHA1.2C_SHA256 and
makes at least certain Debian versions choke (Ubuntu seems not to mind).
Fixes #51.
Merge pull request #52 from antaflos/fix_sha_checksum_labels

Change SHA field labels to SHA1/SHA256 in Release file