Roll back `npm-install-webpack-plugin` version
Roll back `npm-install-webpack-plugin` version

The latest (`3.1.0`) version apparently causes some strange behavior when interacting with webpack dev server (see this issue: https://github.com/ericclemmons/npm-install-webpack-plugin/issues/43).

As a result, hard-coding the version number for now so newly-generated apps will serve correctly.

cafreeman/generator-mobx-react,cafreeman/generator-mobx-react