bug fix for "puts undefined error"

Author: szymonkudzia

lib/index.js

@@ -193,7 +193,7 @@ var WebpackShellPlugin = function () {
     key: 'handleScript',
     value: function handleScript(script) {
       if (os.platform() === 'win32') {
-        this.spreadStdoutAndStdErr(exec(script, puts));
+        this.spreadStdoutAndStdErr(exec(script, this.puts));
       } else {
         var _serializeScript = this.serializeScript(script),
             command = _serializeScript.command,
@@ -273,4 +273,4 @@ var WebpackShellPlugin = function () {
   return WebpackShellPlugin;
 }();
 
-module.exports = WebpackShellPlugin;
+module.exports = WebpackShellPlugin;