[TEAM] Modifying the minimum required RAM

[flocculant]

As per 
https://lists.ubuntu.com/archives/xubuntu-devel/2017-January/011416.html

While I have no doubt that Xubuntu will 'run' with 512MB of RAM - I'd 
not personally like to do so with the increasing requirement that 
applications (browsers for instance) place upon a system.

Even a vm Xubuntu has used 305MB according to free -m without anything 
started.

Better to up that minimum to at least 1024MB.