Update link to Gammapy mailing list
.. _gamma-sky.net: http://gamma-sky.net/
.. _Gammapy binder: http://mybinder.org/repo/gammapy/gammapy-extra
-.. _Gammapy mailing list: http://groups.google.com/group/gammapy
+.. _Gammapy mailing list: https://groups.google.com/forum/#!forum/gammapy
.. _Gammapy Github page: https://github.com/gammapy/gammapy
.. _Gammapy documentation: http://docs.gammapy.org/
.. _Gammapy project summary on Open HUB: https://www.openhub.net/p/gammapy
