ci: pin webpack version

Author: yyx990803

package.json

@@ -137,7 +137,7 @@
     "shelljs": "^0.8.1",
     "terser": "^3.10.2",
     "typescript": "^3.1.3",
-    "webpack": "^4.29.5",
+    "webpack": "~4.28.4",
     "weex-js-runtime": "^0.23.6",
     "weex-styler": "^0.3.0",
     "yorkie": "^2.0.0"