Update install doc to use v0.5 branch

Closes: #673

docs/installation_instructions.rst

@@ -66,18 +66,13 @@ Getting the installation script
 -------------------------------
 
 On your **provisioning machine**, clone the OpenAppStack git repository
-and checkout the latest tagged version (currently ``0.4.0``):
+and checkout the latest release branch (currently ``v0.5``):
 
 ::
 
-    $ git clone -b 0.4.0 https://open.greenhost.net/openappstack/openappstack.git
+    $ git clone -b v0.5 https://open.greenhost.net/openappstack/openappstack.git
     $ cd openappstack
 
-.. note::
-  Git will display a warning after you use the ``git`` command saying that you
-  are in a *detached HEAD* state. This is perfectly normal and you can proceed
-  without any further actions.
-
 Create a python virtual environment called "env" that uses python 3.
 This makes sure we do not change any of your other python projects. The
 second command "activates" the virtualenv.