chore(deps): update dependency postcss-cli to v6 (#381)

Author: renovate[bot]

package-lock.json

@@ -63,7 +63,7 @@
     "mversion": "1.12.0",
     "node-sass": "4.9.2",
     "onchange": "4.1.0",
-    "postcss-cli": "5.0.1",
+    "postcss-cli": "6.0.0",
     "prettier": "1.14.0",
     "pretty-bytes-cli": "2.0.0",
     "semver": "5.5.0",