Add missing webpack-dev-server devDependency

[jaythomas]

Seems the grunt task depends on webpack-dev-server, because installing it resolved the issue for me.

To replicate the issue:

1. Clean the repository of any prior installed node_modules and other changes (git clean -xfd)
2. Make sure webpack and webpack-dev-server node modules aren't globally installed
3. Run npm install
4. Run npm run dev

Results: This error will occur...

[dhilt]

Looks like this is grunt-webpack package issue: https://stackoverflow.com/questions/39364017/grunt-webpack-cannot-find-module-webpack-dev-server. We will add this dependency as a temporary solution, thanks.

[jaythomas]

If you update to the latest version of grunt-webpack, it appears it is now made an optional dependency: webpack-contrib/grunt-webpack#70

But since it is required for local development and testing here I think it is best left explicitly defined as a devDependency.

[dhilt]

Well, I have updated grunt-webpack and webpack itself, and this has allowed me to remove webpack-dev-server from the project. We are using grunt-contrib-connect for dev server and no webpack-dev-server is needed; the only trouble was in that log-warning. Now the project is clear. Thanks!