chore(package): update postcss-cli to version 7.1.0

Closes #137

Author: greenkeeper[bot]

package.json

@@ -66,7 +66,7 @@
     "mkdirp": "^0.5.1",
     "nyc": "^13.1.0",
     "postcss-at-rules-variables": "^0.1.6",
-    "postcss-cli": "^6.1.1",
+    "postcss-cli": "^7.1.0",
     "postcss-custom-properties": "7.0.0",
     "postcss-discard-comments": "^4.0.1",
     "postcss-each": "^0.10.0",