libsodium: Updates source URL. (#10483)

The source URL for versions 1.0.15 and older is updated to reflect where
those archives are now hosted by the upstream libsodium developers.

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

@@ -28,6 +28,6 @@ def url_for_version(self, version):
         url = 'https://download.libsodium.org/libsodium/releases/'
         if version < Version('1.0.4'):
             url += 'old/unsupported/'
-        elif version < Version('1.0.12'):
+        elif version < Version('1.0.16'):
             url += 'old/'
         return url + 'libsodium-{0}.tar.gz'.format(version)