Update grunt to use local phantomjs and quicker diffing

Previously, `grunt prepare` and `grunt diff` used a system version of PhantomJS but one is installed by `css-diff` anyway.

Also, since "baking" took a long time, it was split into 2 steps:
- all the fancy non-browser-supported stuff (ie `target-counter(...)`, `string-set:`)
- all the simple css (borders, fonts, colors)

but it doesn't anymore.