Update pypi.python.org URL to pypi.org
For details on the new PyPI, see the blog post:
Install
=======
-The package is hosted on `PyPI <http://pypi.python.org/pypi/lz4>`_ and so can be
+The package is hosted on `PyPI <https://pypi.org/project/lz4/>`_ and so can be
installed via pip::
$ pip install lz4
