use svg (tensorpack#881)

* use svg

I can't look at it anymore.

* Update README.md

use gitter brand color #ed1965 or #46bc99

README.md

@@ -4,9 +4,8 @@ Tensorpack is a neural network training interface based on TensorFlow.
 
 [![Build Status](https://travis-ci.org/tensorpack/tensorpack.svg?branch=master)](https://travis-ci.org/tensorpack/tensorpack)
 [![ReadTheDoc](https://readthedocs.org/projects/tensorpack/badge/?version=latest)](http://tensorpack.readthedocs.io/en/latest/index.html)
-[![Gitter chat](https://badges.gitter.im/gitterHQ/gitter.png)](https://gitter.im/tensorpack/users)
+[![Gitter chat](https://img.shields.io/badge/chat-on%20gitter-46bc99.svg)](https://gitter.im/tensorpack/users)
 [![model-zoo](https://img.shields.io/badge/model-zoo-brightgreen.svg)](http://models.tensorpack.com)
-
 ## Features:
 
 It's Yet Another TF high-level API, with __speed__, __readability__ and __flexibility__ built together.