refactor: Chunk.modules deprecation warning

 BREAKING CHANGE: Updates to `Chunk.mapModules`. This release is not backwards compatible with `Webpack 2.x` due to breaking changes in webpack/webpack#4764

Author: joshwiens

src/index.js

@@ -144,7 +144,7 @@ class ExtractTextPlugin {
         async.forEach(chunks, (chunk, callback) => { // eslint-disable-line no-shadow
           const extractedChunk = extractedChunks[chunks.indexOf(chunk)];
           const shouldExtract = !!(options.allChunks || isInitialOrHasNoParents(chunk));
-          async.forEach(chunk.modules.slice(), (module, callback) => { // eslint-disable-line no-shadow
+          async.forEach(chunk.mapModules((c) => { return c; }), (module, callback) => { // eslint-disable-line no-shadow, arrow-body-style
             let meta = module[NS];
             if (meta && (!meta.options.id || meta.options.id === id)) {
               const wasExtracted = Array.isArray(meta.content);