checksums: Remove or fix up unfetchable or unverifiable md5 versions

This commit either removes remaining md5 checksums, or it replaces them
with sha256 checksums verified by fetching over https.

var/spack/repos/builtin/packages/cub/package.py

@@ -14,8 +14,6 @@ class Cub(Package):
     url      = "https://github.com/NVlabs/cub/archive/1.6.4.zip"
 
     version('1.7.1', sha256='50b8777b83093fdfdab429a61fccdbfbbb991b3bbc08385118e5ad58e8f62e1d')
-    version('1.6.4', md5='924fc12c0efb17264c3ad2d611ed1c51')
-    version('1.4.1', md5='74a36eb84e5b5f0bf54aa3df39f660b2')
 
     def install(self, spec, prefix):
         mkdirp(prefix.include)