fix: relax webpack version requirement

To avoid introducing two different instances of webpack when
@cypress/webpack-preprocessor 3.0.0/3.0.1 is also installed, which
breaks HMR.

Fixes #2873
Fixes #2892

packages/@vue/cli-service/package.json

@@ -65,7 +65,7 @@
     "thread-loader": "^1.2.0",
     "url-loader": "^1.1.2",
     "vue-loader": "^15.4.2",
-    "webpack": "^4.23.1",
+    "webpack": "^4.18.1",
     "webpack-bundle-analyzer": "^3.0.3",
     "webpack-chain": "^4.11.0",
     "webpack-dev-server": "^3.1.10",