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.