fix: client/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-JSYAML-173999
- https://snyk.io/vuln/SNYK-JS-JSYAML-174129
- https://snyk.io/vuln/npm:mem:20180117

client/package.json

@@ -12,7 +12,7 @@
   },
   "dependencies": {
     "clean-webpack-plugin": "^0.1.17",
-    "css-loader": "^0.28.4",
+    "css-loader": "^1.0.0",
     "html-webpack-plugin": "^2.28.0",
     "path": "^0.12.7",
     "react": "^15.6.1",
@@ -30,7 +30,7 @@
     "redux-thunk": "^2.2.0",
     "urijs": "^1.18.12",
     "webfontloader": "^1.6.28",
-    "webpack": "^3.5.5",
+    "webpack": "^4.0.0",
     "webpack-dev-server": "^3.1.11",
     "webpack-merge": "^4.1.0"
   },