chore(package): update postcss-csso to version 2.0.0

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -72,7 +72,7 @@
     "postcss-clearfix": "^1.0.0",
     "postcss-cli": "^2.6.0",
     "postcss-conditionals": "^2.1.0",
-    "postcss-csso": "^1.1.2",
+    "postcss-csso": "^2.0.0",
     "postcss-custom-properties": "^5.0.2",
     "postcss-devtools": "^1.1.1",
     "postcss-each": "^0.9.3",