Upgrade PostCSS to 7.0.2

Author: Aaron Ballard

plugins/postcss-env-function/package.json

@@ -25,7 +25,7 @@
     "node": ">=4.0.0"
   },
   "dependencies": {
-    "postcss": "^6.0.22",
+    "postcss": "^7.0.2",
     "postcss-values-parser": "^1.5.0"
   },
   "devDependencies": {