Remove cssnano from dev dependencies

Author: mrmrs

package.json

@@ -21,7 +21,6 @@
     "css": "^2.1.0",
     "css-mqpacker": "^4.0.0",
     "css-parse": "^2.0.0",
-    "cssnano": "^3.3.1",
     "cssstats": "^2.0.1",
     "filesize": "^3.1.3",
     "get-classes-from-html": "^1.0.1",