Updated installation.rst to new pip source url

The old pip url was outdated as evidenced by the contents of the file downloaded. Used the new url that the file pointed to.

docs/source/installation.rst

@@ -239,7 +239,7 @@ Then::
 
   cd /home/mozdef
 
-  wget https://raw.github.com/pypa/pip/master/contrib/get-pip.py
+  wget https://bootstrap.pypa.io/get-pip.py
   export LD_LIBRARY_PATH=/home/mozdef/python2.7/lib/
   ./python2.7/bin/python get-pip.py
   ./python2.7/bin/pip install virtualenv