Update script doesn't work without internet connection

[afuersch]

The problem is the used extract zip module, which is an external dependency to a script in this repository. Remove the module by copying the functionality into the update script, to make the script work without internet connection.

[bwolkerstorfer]

@afuersch Why do we need a special script that does the ZIP interaction? PS has the functionality built-in, see https://docs.microsoft.com/de-de/powershell/module/Microsoft.PowerShell.Archive/Expand-Archive?view=powershell-5.1

[afuersch]

@bwolkerstorfer Currently the update-script uses the https://github.com/eRecruiter/eRecruiter.PowerShell/blob/master/tools/Extract-Zip.ps1 script for extracting the ZIP archive. This should be replaced by function you mentioned, since the script doesn't use it either.