fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-JSON5-3182856

Author: snyk-bot

package.json

@@ -42,7 +42,7 @@
     "eslint-loader": "1.4.1",
     "eslint-plugin-import": "1.10.3",
     "eslint-plugin-react": "5.2.2",
-    "extract-text-webpack-plugin": "1.0.1",
+    "extract-text-webpack-plugin": "2.0.0",
     "file-loader": "0.9.0",
     "fs-extra": "^0.30.0",
     "html-webpack-plugin": "2.22.0",