Fix dead link in the documentation [issue 294]

Author: evertrol

docs/usage.rst

@@ -4,16 +4,13 @@ Quick Start
 Install
 -------
 
-This package is available on `PyPI`_ and `Crate.io`_.
-
-Install from PyPI with ``pip``:
+Install from `PyPI`_ with ``pip``:
 
 .. code-block:: bash
 
     $ pip install django-simple-history
 
 .. _pypi: https://pypi.python.org/pypi/django-simple-history/
-.. _crate.io: https://crate.io/packages/django-simple-history/
 
 
 Configure