Updated link to Tox documentation in tox.ini.
-# Tox (http://tox.testrun.org/) is a tool for running tests in multiple
+# Tox (https://tox.readthedocs.io/) is a tool for running tests in multiple
# virtualenvs. This configuration file helps to run the test suite on all
# supported Python versions. To use it, "pip install tox" and then run "tox"
# from this directory.
