Fix Chunk.modules DeprecationWarning in Webpack 3.x

Updates to Chunk.mapModules is not backwards compatible with Webpack 2.x due to breaking changes in webpack/webpack#4764

Following should be added to `README`:

### Install
```
# for webpack 3
npm install --save-dev extract-text-webpack-plugin
# for webpack 1, 2
npm install --save-dev extract-text-webpack-plugin@0.1.0
```

Plugin.js

@@ -1,8 +1,9 @@
-'use strict';
-
 class ExtractFilePlugin {
+
     apply(compiler) {
+
         compiler.plugin('this-compilation', (compilation) => {
+
             compilation.plugin('normal-module-loader', (loaderContext, module) => {
                 loaderContext[__dirname] = () => {
                     module.meta[__dirname] = true;
@@ -11,7 +12,7 @@ class ExtractFilePlugin {
 
             compilation.plugin('additional-assets', callback => {
                 compilation.chunks.forEach(chunk => {
-                    chunk.modules.forEach(module => processModule(chunk, module));
+                    chunk.forEachModule(module => processModule(chunk, module));
                 });
 
                 callback();
@@ -21,22 +22,28 @@ class ExtractFilePlugin {
 }
 
 function processModule(chunk, ourModule) {
+
     if (ourModule.meta && ourModule.meta[__dirname]) {
+
+        let moduleFound = false;
+
         // let's find module, which was issued by ours (proxied module)
-        chunk.modules.some((module) => {
-            if (module.reasons.some(reason => reason.module === ourModule)) {
+        chunk.forEachModule((module) => {
+
+            if (!moduleFound && module.reasons.some(reason => reason.module === ourModule)) {
                 // add assets from that module
                 addAssets(chunk, module);
                 // break cycle
-                return true;
+                moduleFound = true;
             }
         });
     }
 }
 
 function addAssets(chunk, module) {
+
     // add any emitted assets via proxied module to this chunk
-    for (var file in module.assets) {
+    for (let file in module.assets) {
         chunk.files.push(file);
     }
 }