fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-MERGE-1042987

Author: snyk-bot

package.json

@@ -36,7 +36,7 @@
     "gzip-size": "3.0.0",
     "html-webpack-plugin": "2.24.0",
     "http-proxy-middleware": "0.17.3",
-    "jest": "18.1.0",
+    "jest": "24.0.0",
     "json-loader": "0.5.4",
     "object-assign": "4.1.1",
     "postcss-import": "^9.1.0",