Set webpack-dev-middleware to fixed version

[emplums]

Fixes: #119

Since webpack-dev-middleware version 3.6.1 has been released, koa-webpack (imported by webpack-serve) breaks with that version and running x0 locally doesn't work. Since webpack-serve is using non-fixed versions, and so is koa-webpack I think the easiest fix here is to just set webpack-dev-middleware to a fixed version to avoid the issue. It adds another dependency that technically doesn't need to be there, but since webpack-serve is no longer maintained, it might be the best bet.