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

@@ -1,44 +1,51 @@
-'use strict';
-
 class ExtractFilePlugin {
-    apply(compiler) {
-        compiler.plugin('this-compilation', (compilation) => {
-            compilation.plugin('normal-module-loader', (loaderContext, module) => {
-                loaderContext[__dirname] = () => {
-                    module.meta[__dirname] = true;
-                };
-            });
-
-            compilation.plugin('additional-assets', callback => {
-                compilation.chunks.forEach(chunk => {
-                    chunk.modules.forEach(module => processModule(chunk, module));
-                });
-
-                callback();
-            });
-        });
-    }
+
+	apply(compiler) {
+
+		compiler.plugin('this-compilation', (compilation) => {
+
+			compilation.plugin('normal-module-loader', (loaderContext, module) => {
+				loaderContext[__dirname] = () => {
+					module.meta[__dirname] = true;
+				};
+			});
+
+			compilation.plugin('additional-assets', callback => {
+				compilation.chunks.forEach(chunk => {
+					chunk.forEachModule(module => processModule(chunk, module));
+				});
+
+				callback();
+			});
+		});
+	}
 }
 
 function processModule(chunk, ourModule) {
-    if (ourModule.meta && ourModule.meta[__dirname]) {
-        // let's find module, which was issued by ours (proxied module)
-        chunk.modules.some((module) => {
-            if (module.reasons.some(reason => reason.module === ourModule)) {
-                // add assets from that module
-                addAssets(chunk, module);
-                // break cycle
-                return true;
-            }
-        });
-    }
+
+	if (ourModule.meta && ourModule.meta[__dirname]) {
+
+		let moduleFound = false;
+
+		// let's find module, which was issued by ours (proxied module)
+		chunk.forEachModule((module) => {
+
+			if (!moduleFound && module.reasons.some(reason => reason.module === ourModule)) {
+				// add assets from that module
+				addAssets(chunk, module);
+				// break cycle
+				moduleFound = true;
+			}
+		});
+	}
 }
 
 function addAssets(chunk, module) {
-    // add any emitted assets via proxied module to this chunk
-    for (var file in module.assets) {
-        chunk.files.push(file);
-    }
+
+	// add any emitted assets via proxied module to this chunk
+	for (let file in module.assets) {
+		chunk.files.push(file);
+	}
 }
 
 module.exports = ExtractFilePlugin;