fix(package): update postcss to version 7.0.0

Author: greenkeeper[bot]

package.json

@@ -46,7 +46,7 @@
   "homepage": "https://www.purgecss.com",
   "dependencies": {
     "glob": "^7.1.2",
-    "postcss": "^6.0.23",
+    "postcss": "^7.0.0",
     "postcss-selector-parser": "^5.0.0-rc.3",
     "yargs": "^12.0.1"
   },