feat: add webpack as a devDependency

[mattlewis92]

Continuing the discussion from webpack-contrib/less-loader#208 (comment)

I believe webpack should be included as a dev dependency because:

• checking out a webpack-contrib repo, running npm install and npm t should just work. Instead you're greeted with this error message, it's not immediately obvious that webpack needs to be installed with npm install webpack. Some users will just give up here and not bother contributing to the repo.

• secondly once running npm install webpack to get the tests running, because of the npm 5 requirement, package.json and package-lock.json are automatically updated and will appear in the git diff before the user has even made any changes. Based on my own experience with open source, people will commit these changes in a pull request, and it's going to lead to maintainers asking contributors to remove these files in reviews which is just a waste of everyones time and energy.

I can't really see any advantages to not having it installed as a dev dependency, travis already automatically tests the repo against all supported versions of webpack and will catch the rare time something is broken on an older version of webpack.

Closes #78

[joshwiens]

I think the general consensus after talking about this in Slack is we'll add it as a devDep & move the burden of explicit versions / dev versions / whatever onto the maintainers.

My only concern, if you can call it that is given we have 51 maintained repos and climbing this should probable be set to latest as well.

It's a pipe dream to think we are going to have time / always remember to bump the dev dependency to keep up with Webpack releases.

@mattlewis92 - DM me a email on Twitter ( same name ), i'll get you a slack invite.