gulp-minify-css removes `px` values

[flovan]

The rem mixin generates pixel fallbacks for older browsers, though when IE8 or lower needs to be supported, gulp-minify-css strips these out, leaving only the rem values and a crippled IE8 in its wake.

Current fix:

.pipe(plugins.minifyCss({ compatibility: 'ie8', noAdvanced: true }))

Think about exposing these through a supportOldie config option, and keep an eye on clean-css/clean-css#375 so noAdvanced can be removed.