Generate SHA-512 checksums for releases

This commit makes it so that we upload SHA-512 checksums instead of
MD5 checksums with release tarballs.

Author: DirectXMan12

.travis.before-deploy.sh

@@ -8,4 +8,4 @@ mkdir ./tag_build
 
 # create and checksum the tarball
 tar -czvf ./tag_build/${TRAVIS_TAG}.tar.gz --exclude='tag_build' --exclude='.git' --transform "s,^\.,python-gssapi-${TRAVIS_TAG}," .
-md5sum --binary ./tag_build/${TRAVIS_TAG}.tar.gz > ./tag_build/${TRAVIS_TAG}.md5sum
+sha512sum --binary ./tag_build/${TRAVIS_TAG}.tar.gz > ./tag_build/${TRAVIS_TAG}.sha512sum