fix: use new tapable webpack API

Author: patrickhulce

lib/index.js

@@ -121,8 +121,8 @@ class NukeCssPlugin {
   }
 
   apply(compiler) {
-    compiler.plugin('this-compilation', compilation => {
-      compilation.plugin('additional-assets', done => {
+    compiler.hooks.thisCompilation.tap('NukecssPlugin', compilation => {
+      compilation.hooks.additionalAssets.tapAsync('NukecssPlugin', done => {
         this.onAdditionalAssets(compilation, done)
       })
     })