message: Fix invaild gitter url
diff: @@ -6,7 +6,7 @@ Superset
  [![Coverage Status](https://coveralls.io/repos/apache/incubator-superset/badge.svg?branch=master&service=github)](https://coveralls.io/github/apache/incubator-superset?branch=master)
  [![PyPI](https://img.shields.io/pypi/pyversions/superset.svg?maxAge=2592000)](https://pypi.python.org/pypi/superset)
  [![Requirements Status](https://requires.io/github/apache/incubator-superset/requirements.svg?branch=master)](https://requires.io/github/apache/incubator-superset/requirements/?branch=master)
  -[![Join the chat at https://gitter.im/apache/incubator-superset](https://badges.gitter.im/apache/incubator-superset.svg)](https://gitter.im/apache/incubator-superset?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
  +[![Join the chat at https://gitter.im/airbnb/superset](https://badges.gitter.im/apache/incubator-superset.svg)](https://gitter.im/airbnb/superset?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
  [![Documentation](https://img.shields.io/badge/docs-apache.org-blue.svg)](https://superset.incubator.apache.org)
  [![dependencies Status](https://david-dm.org/apache/incubator-superset/status.svg?path=superset/assets)](https://david-dm.org/apache/incubator-superset?path=superset/assets)
  