[plasma-workspace] /: Bump minimum requirement to Qt 5.6.0

[Marco Martin]

On Monday 30 May 2016, Aleix Pol wrote:
> > Otherwise, I'm really glad we finally got the plasmashell goes QScreen
> > in, thanks a lot for the perseverance!
> 
> Putting a 5.6.1 there will make it not compile for anyone, if they
> have Qt by the distro.

i think that's actually fine, as forces us to complain to qt more loudly
-- 
Marco Martin