chore(package): update postcss-custom-properties to version 7.0.0

Author: greenkeeper[bot]

package.json

@@ -63,7 +63,7 @@
     "nyc": "^11.3.0",
     "postcss-at-rules-variables": "^0.1.1",
     "postcss-cli": "^5.0.0",
-    "postcss-custom-properties": "^6.1.0",
+    "postcss-custom-properties": "^7.0.0",
     "postcss-discard-comments": "^4.0.0-rc.2",
     "postcss-each": "^0.10.0",
     "prettier": "^1.7.4",