*: Using sha512 instead of sha256

sha512 is faster on 64bit systems than sha256, so prefer it over sha256

fixes #187 
Oh you're right, I misunderstood what @philips was saying in his other comment.

I'm not familiar with how ```-r``` works.  What's the typical way for updating packages this way?  There is no ```godep update -r <package>```, and using ```godep save -r <package>``` wipes out all the other packages.