PowerShellVersion should be required for modules

[juneb]

Users need to know the PS version requirements for a module. To specify, add a value for PowerShellVersion key in the module manifest.

To verify, get the manifest:
iex (cat -raw (gmo -list ).Path)

Or PSModuleInfo
(gmo -list ).PowerShellVersion

[KirkMunro]

I think it would be best if lack of a PowerShellVersion attribute generated a warning that the PowerShellVersion attribute should be added for any modules that require PowerShell 3.0 or later. Legacy modules written for PowerShell 2.0 that work as is without a PowerShellVersion attribute in the manifest shouldn't be forced to have a manifest update when it is actually not required (hence the warning).

[joeyaiello]

This brings up an interesting point about modules compatible with v2: we should probably provide a default profile that targets modules which intend to be compatible with v2.