add hint for missing package on Debian Wheezy

the python-augeas itself does wrongly not depend on libpython2.7, so if
libpython2.7 is not yet installed, an 'import augeas' in python fails.
This could easily be solved by manually installing the missing package.

Hope this helps anybody, since I'm new to ansible, it took me some time to find out what's the cause of this module "not working" ;)