Upgrade PostCSS to 7.0.2

Author: Aaron Ballard

plugins/postcss-focus-visible/package.json

@@ -24,7 +24,7 @@
     "node": ">=4.0.0"
   },
   "dependencies": {
-    "postcss": "^6.0"
+    "postcss": "^7.0.2"
   },
   "devDependencies": {
     "babel-core": "^6.26",