Decide what to do about old PowerShell versions

[carolynvs]

We either need doc and detect the min version or support it.

See https://github.com/getcarina/carina/issues/83

[carolynvs]

Here's how to verify a minimum version

if($PSVersionTable -eq $null -or $PSVersionTable.PSVersion.Major -lt 4){
  Write-Output "dvm requires PowerShell version 4 or higher."
  Exit 1
}