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.
Update HISTORY and README for 0.4.1, the new 0.4.0