chore(deps): update dependency postcss-loader to v8

Author: renovate[bot]

package.json

@@ -76,7 +76,7 @@
     "lodash.defaultsdeep": "4.6.1",
     "mkdirp": "1.0.4",
     "postcss-import": "12.0.1",
-    "postcss-loader": "3.0.0",
+    "postcss-loader": "8.0.0",
     "postcss-simple-vars": "5.0.2",
     "raf": "3.4.1",
     "react": "16.2.0",