*: Using sha512 instead of sha256

sha512 is faster on 64bit systems than sha256, so prefer it over sha256

fixes #187 
Hrm, this isn't right :/. We currently use rewrites for all imports (and hence build without godep), and we should continue to do that for everything (or switch _entirely_ over to un-rewritten imports, but that would be a separate issue).

Did you use `-r` when bumping the dep? Not sure why it did not rewrite the paths there properly...