checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

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

@@ -14,7 +14,7 @@ class Igvtools(Package):
     homepage = "https://software.broadinstitute.org/software/igv/home"
     url      = "http://data.broadinstitute.org/igv/projects/downloads/2.3/igvtools_2.3.98.zip"
 
-    version('2.3.98', '17ed12a213cd9cdaeb2f2a5c56d84bc4')
+    version('2.3.98', sha256='07027c179f25960bab9919f255c0f8e08f0861861ac6dc02d92be8313e0316a4')
 
     depends_on('java@8:')