Update postcss to version 7.0.0 (postcss#229)

Author: greenkeeper[bot]

package.json

@@ -24,7 +24,7 @@
     "fs-extra": "^5.0.0",
     "get-stdin": "^6.0.0",
     "globby": "^8.0.0",
-    "postcss": "^6.0.1",
+    "postcss": "^7.0.0",
     "postcss-load-config": "^1.1.0",
     "postcss-reporter": "^5.0.0",
     "pretty-hrtime": "^1.0.3",