Update dependency image-webpack-loader to v4

Author: renovate-bot

package.json

@@ -56,7 +56,7 @@
     "eslint-plugin-react": "7.5.1",
     "extract-text-webpack-plugin": "3.0.2",
     "file-loader": "0.11.2",
-    "image-webpack-loader": "3.6.0",
+    "image-webpack-loader": "4.3.1",
     "jest": "21.2.1",
     "json-loader": "0.5.7",
     "markdown-it-anchor": "4.0.0",