powershell installation doesn't work on PS v4 or below

[Voileexperiments]

On PS v4 or below Expand-Archive is unavailable, so the install script fails.

This seems to work (I haven't tested this intensively):

Add-Type -assembly "system.io.compression.filesystem"
[System.IO.Compression.ZipFile]::ExtractToDirectory("$_dir/elan-init.zip", "$_dir")