Powershell 5 compatibility for Build-Module.ps1

[sevaa]

PR Summary

Encoding UTF8BOM for Set-Content was not available is PS5. This PR makes the encoding value conditional.

The readme states the lower bound of Powershell compatibility at 5.

The only way to test that is to introduce an explicitly PS5 build in the CI scripts. That's a maintainer level decision.

The CI will fail over the Update-VSTeamProject autotest issue. Only merge when/if either #494 or #495 lands.

PR Checklist

• [ ] Write Help
• [ ] Write Unit Test
• [ ] Update CHANGELOG.md

[sevaa]

Told you so :)

[SebastianSchuetze]

I can only merge when the test do not fail.

[sevaa]

Builds now.