Updated link to Tox documentation in tox.ini.

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.