Switch from pyup.io to Dependabot (#153)

Author: stephendonner

README.rst

@@ -17,9 +17,9 @@ axe-selenium-python integrates aXe and selenium to enable automated web accessib
 .. image:: https://img.shields.io/github/issues-raw/mozilla-services/axe-selenium-python.svg
    :target: https://github.com/mozilla-services/axe-selenium-python/issues
    :alt: Issues
-.. image:: https://pyup.io/repos/github/mozilla-services/axe-selenium-python/shield.svg
-   :target: https://pyup.io/repos/github/mozilla-services/axe-selenium-python/
-   :alt: Updates
+.. image:: https://api.dependabot.com/badges/status?host=github&repo=mozilla-services/axe-selenium-python
+   :target: https://dependabot.com
+   :alt: Dependabot
 
 
 Requirements