update docs to point to june-allies branch of beat.tutorial for install, closes #150

doc/install.rst

@@ -26,6 +26,10 @@ Installation Steps
 
    $ git clone https://gitlab.idiap.ch/beat/beat.tutorial.git
 
+#. Checkout the Git branch containing the packer scripts: ::
+
+   $ git checkout june-allies
+
 #. Change directory to the folder holding the scripts: ::
 
    $ cd beat.tutorial/packer_scripts