STRF-5934: Lock down webpack version

* Webpack 4.29.0 appears to have introduced a breaking change
  from the 5.0 branch.
* Lock down versions to the ones used during development of #1390

Author: mattolson

package.json

@@ -57,10 +57,10 @@
     "lodash-webpack-plugin": "^0.11.2",
     "npx": "^10.2.0",
     "time-grunt": "^1.2.2",
-    "webpack": "^4.27.1",
+    "webpack": "~4.27.1",
     "webpack-bundle-analyzer": "^3.0.3",
-    "webpack-cli": "^3.1.2",
-    "webpack-merge": "^4.1.2"
+    "webpack-cli": "~3.1.2",
+    "webpack-merge": "~4.1.2"
   },
   "scripts": {
     "build": "npx webpack --config webpack.prod.js",