fix image issue

Author: zeyiwen

README.md

@@ -1,6 +1,6 @@
 [![Build Status](https://travis-ci.org/Xtra-Computing/thundersvm.svg?branch=master)](https://travis-ci.org/zeyiwen/thundersvm)
 [![Build status](https://ci.appveyor.com/api/projects/status/e9yoehx7orsrsh89/branch/master?svg=true)](https://ci.appveyor.com/project/shijiashuai/thundersvm/branch/master)
-[![GitHub license](http://dmlc.github.io/img/apache2.svg)](./LICENSE)
+[![GitHub license](https://img.shields.io/badge/license-apache2-yellowgreen)](./LICENSE)
 [![Documentation Status](https://readthedocs.org/projects/thundersvm/badge/?version=latest)](https://thundersvm.readthedocs.org)
 [![GitHub issues](https://img.shields.io/github/issues/Xtra-Computing/thundersvm.svg)](https://github.com/zeyiwen/thundersvm/issues)
 [![PyPI version](https://badge.fury.io/py/thundersvm.svg)](https://badge.fury.io/py/thundersvm)