updated ubuntu instructions

Author: Kenneth Reitz

docs/starting/installation.rst

@@ -128,6 +128,7 @@ Natty Narwhal, the latest version of Ubuntu, **comes with Python 2.7 out of the
 
 Older versions of Python aren't available from the official repository. However, if it's needed (for example to support legacy code), we can add an unsupported repository and install an older version of Python (2.5 in the example below)::
 
+    $ sudo apt-get install python-software-properties
     $ sudo add-apt-repository ppa:fkrull/deadsnakes
     $ sudo apt-get update
     $ sudo apt-get install python2.5