Set min required PowerShell version to 4

[Eilon]

To avoid people with Windows 7 and PowerShell 3 trying to run the build scripts.

[mikes-gh]

I was the one with the above issue. It was a proxy server issue in the end. I upgraded to PS 5 by the time I realised the problem but I suspect PS 3 would still work fine.
I think dotnet restore could come back with a better error if nugget sources aren't available.