Implement longer-term fix for #1066 (see description)

- declare explict @types/react-router version
- update to newer aspnet-webpack-react that doesn't include the hacky
workaround since it's not needed if you have the correct
@types/react-router version

In a subsequent commit, will also shrinkwrap to pin all dependency
versions. This isn't strictly required right now, but is the right thing
for long-term stability.

templates/ReactSpa/package.json

@@ -8,7 +8,7 @@
     "@types/react-router": "4.0.9",
     "@types/webpack-env": "1.13.0",
     "aspnet-webpack": "^1.0.29",
-    "aspnet-webpack-react": "^2.0.0",
+    "aspnet-webpack-react": "^3.0.0-beta",
     "awesome-typescript-loader": "3.1.3",
     "bootstrap": "3.3.7",
     "css-loader": "0.28.1",