Downgrade grunt-contrib-cssmin to v0.6.x which uses clean-css v1.x

grunt-conrib-cssmin v0.7.x uses clean-css v2.x which adds "advanced"
options which totally borked the Pure v0.4.0 release.

**Note to self:** If I ever write a trivial Grunt plugin that wraps some
dependency, tie their version releases to the same slot, e.g. bump the
major version number if the dependency's major version number changes.

Gruntfile.js

@@ -115,7 +115,7 @@ grunt.initConfig({
 
     cssmin: {
         options: {
-            // report: 'gzip'
+            noAdvanced: true
         },
 
         files: {

package.json

@@ -10,7 +10,7 @@
     "bower": "~1.2.5",
     "grunt": "~0.4.1",
     "grunt-cli": "~0.1.9",
-    "grunt-contrib-cssmin": "~0.7.0",
+    "grunt-contrib-cssmin": "~0.6.0",
     "grunt-contrib-clean": "~0.5.0",
     "grunt-contrib-copy": "~0.5.0",
     "grunt-contrib-concat": "~0.3.0",