fix(gatsby): Pin webpack to fix gatsby build problem

This addresses gatsbyjs#11198.

There's a regression bug webpack/webpack#8656 affecting 4.29.0 through at least 4.29.3 which causes errors like this during `gatsby build`.

  Error: ./.cache/async-requires.js 8:11
  Module parse failed: Unexpected token (8:11)
  You may need an appropriate loader to handle this file type.
  | exports.components = {
  |   "component---frontend-components-main-js": function componentFrontendCompo  nentsMainJs() {
  >     return import("/home/circleci/project/frontend/components/main.js"
  |     /* webpackChunkName: "component---frontend-components-main-js" */
  |     );
   @ ./.cache/production-app.js 18:0-45 21:23-36 26:23-36

packages/gatsby/package.json

@@ -119,7 +119,7 @@
     "url-loader": "^1.0.1",
     "uuid": "^3.1.0",
     "v8-compile-cache": "^1.1.0",
-    "webpack": "^4.12.0",
+    "webpack": "~4.28.2",
     "webpack-dev-middleware": "^3.0.1",
     "webpack-dev-server": "^3.1.14",
     "webpack-hot-middleware": "^2.21.0",