Update adaptation methods of middlewares in webpack4

[Styx11]

Hi, recently, I have used devMiddleware and hotMiddleware in webback4 and koa2 environment. Your library has greatly inspired me, but I have encountered version problems:

• The middleware version you are using is too old to support webpack4
• The adaptation method designed in the library is no longer applicable to the existing version of middlewares

So in this PR, I upgraded the middleware versions and updated the adaptation methods

I've tested it on my project and they work well🙂

pins:

"koa": "^2.11.0",
"webpack": "^4.42.0",
"webpack-dev-middleware": "^3.7.2",
"webpack-hot-middleware": "^2.25.0",