Update dependency postcss to v7 [SECURITY]

Author: renovate[bot]

package.json

@@ -26,7 +26,7 @@
     "next": "^4.0.3",
     "next-redux-wrapper": "^1.3.4",
     "normalize.css": "^7.0.0",
-    "postcss": "^6.0.11",
+    "postcss": "^7.0.0",
     "react": "^16.0.0",
     "react-dom": "^16.0.0",
     "react-motion": "^0.5.2",