Update dependency css-loader to v6

Author: renovate-bot

package.json

@@ -19,7 +19,7 @@
     "case-sensitive-paths-webpack-plugin": "1.1.2",
     "chalk": "1.1.3",
     "cross-spawn": "4.0.0",
-    "css-loader": "0.23.1",
+    "css-loader": "6.4.0",
     "eslint": "3.1.1",
     "eslint-loader": "3.0.3",
     "eslint-plugin-flowtype": "2.4.0",