Update usage.rst
Replacing broken link.
@@ -6,7 +6,7 @@ Usage
Installation
------------
-Use `pip <http://www.pip-installer.org/en/latest/index.html>`__:
+Use `pip <https://pip.pypa.io/en/stable/installing/>`__:
``$ pip install gmusicapi``.
This will grab all the source dependencies.
Avoid using ``easy_install``.
