chore(package): update postcss-loader to version 3.0.0

Closes #34

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
     "file-loader": "^0.11.0",
     "html-webpack-plugin": "^2.26.0",
     "postcss-cssnext": "^2.7.0",
-    "postcss-loader": "^1.2.2",
+    "postcss-loader": "^3.0.0",
     "regenerator": "^0.10.0",
     "style-loader": "^0.16.0",
     "url-loader": "^0.5.7",