Change CI badge to staging branch

The stable branch is always green.

Author: zhsj

README.rst

@@ -13,7 +13,7 @@ Maintained versions:
      - Build status
      - Translation progress
    * - `3.7 <https://github.com/python/python-docs-zh-cn/tree/3.7>`_
-     - .. image:: https://travis-ci.org/python/python-docs-zh-cn.svg?branch=3.7
+     - .. image:: https://travis-ci.org/python/python-docs-zh-cn.svg?branch=3.7-staging
           :target: https://travis-ci.org/python/python-docs-zh-cn
      - .. image:: https://img.shields.io/badge/dynamic/json.svg?label=zh_CN&query=%24.zh_CN&url=http%3A%2F%2Fgce.zhsj.me%2Fpython/newest
           :target: https://www.transifex.com/python-doc/python-newest/