Update poser badge links on readme (#247)

Author: cgsmith

Diff for: README.md

@@ -12,8 +12,8 @@ For license information check the [LICENSE](LICENSE.md)-file.
 
 Documentation is at [docs/guide/README.md](docs/guide/README.md).
 
-[![Latest Stable Version](https://poser.pugx.org/yiisoft/yii2-httpclient/v/stable.png)](https://packagist.org/packages/yiisoft/yii2-httpclient)
-[![Total Downloads](https://poser.pugx.org/yiisoft/yii2-httpclient/downloads.png)](https://packagist.org/packages/yiisoft/yii2-httpclient)
+[![Latest Stable Version](https://poser.pugx.org/yiisoft/yii2-httpclient/v)](https://packagist.org/packages/yiisoft/yii2-httpclient)
+[![Total Downloads](https://poser.pugx.org/yiisoft/yii2-httpclient/downloads)](https://packagist.org/packages/yiisoft/yii2-httpclient)
 [![Build Status](https://github.com/yiisoft/yii2-httpclient/workflows/build/badge.svg)](https://github.com/yiisoft/yii2-httpclient/actions)