[Plone-docs] update installation.rst to newer versions of ubuntu and debian

[sven]

Hi,

I already sent this yesterday to the installer list, sorry for cross
posting :)
any thoughts or doubts against updating:

https://github.com/collective/collective.developermanual/blob/master/source/getstarted/installation.rst

to Ubuntu 12.04 and Debian 7 ? at the moment the doucument is based on
Debian 5 and Ubuntu 10.04

cheers

Sven