0.7.0 introduces PostCSS issues

[pborrego]

It looks like something in 0.7.0 bumps the PostCSS version for some plugin that requires PostCSS 6.x instead of PostCSS 5.x. And not all PostCSS plugins work with 6.x yet. On my project I had to downgrade cnn-starter-app from 0.7.0 to 0.6.0 to get PostCSS to work. Mostly due to react-toolbox only working with PostCSS 5.x.

At a minimum might want to tag as breaking change instead of minor change or at least make a note about potential issues on PostCSS due to the version changing.