fix(package): update cssnano to version 4.1.8

Closes #34

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
   },
   "bugs": "https://github.com/reshape/minify/issues",
   "dependencies": {
-    "cssnano": "^3.7.4",
+    "cssnano": "^4.1.8",
     "indx": "^0.2.3",
     "reshape-expressions": "^0.1.4",
     "reshape-parser": "^0.3.0",