Issue #18 Push min requirement to Yii 2.0.13

Author: mikehaertl

README.md

@@ -6,6 +6,8 @@ Yii2 Excel Export
 [![Latest Unstable Version](https://poser.pugx.org/codemix/yii2-excelexport/v/unstable)](https://packagist.org/packages/codemix/yii2-excelexport)
 [![License](https://poser.pugx.org/codemix/yii2-excelexport/license)](https://packagist.org/packages/codemix/yii2-excelexport)
 
+> **Note:** The minimum requirement since 2.6.0 is Yii 2.0.13. The latest
+> version for older Yii releases is 2.5.0.
 
 ## Features
 

composer.json

@@ -11,7 +11,7 @@
         }
     ],
     "require": {
-        "yiisoft/yii2": "*",
+        "yiisoft/yii2": "~2.0.13",
         "mikehaertl/php-tmpfile": "^1.0.0",
         "phpoffice/phpexcel": "1.*"
     },

src/ExcelFile.php

@@ -2,14 +2,14 @@
 namespace codemix\excelexport;
 
 use Yii;
-use yii\base\Object;
+use yii\base\BaseObject;
 use yii\helpers\ArrayHelper;
 use mikehaertl\tmp\File;
 
 /**
  * This class represents an excel file.
  */
-class ExcelFile extends Object
+class ExcelFile extends BaseObject
 {
     /**
      * @var string the writer class to use. Default is