Use non-deprecated webpack 4 API's

Author: SpaceK33z

lib/Server.js

@@ -65,9 +65,9 @@ function Server(compiler, options) {
     });
     compiler.apply(progressPlugin);
   }
-  compiler.plugin('compile', invalidPlugin);
-  compiler.plugin('invalid', invalidPlugin);
-  compiler.plugin('done', (stats) => {
+  compiler.hooks.compile.tap('webpack-dev-server', invalidPlugin);
+  compiler.hooks.invalid.tap('webpack-dev-server', invalidPlugin);
+  compiler.hooks.done.tap('webpack-dev-server', (stats) => {
     this._sendStats(this.sockets, stats.toJson(clientStats));
     this._stats = stats;
   });