Agent folder overwrite support throws in DSC 1.1

[HamsterExAstris]

#43 added support for overwriting an existing agent folder, but the ZipFile.ExtractToDirectory overload it calls only exists on .NET Core; this means it fails when hitting that code branch in DSC 1.1, which still uses .NET Framework.

Cannot convert argument "entryNameEncoding", with value: "True", for "ExtractToDirectory" to type
"System.Text.Encoding": "Cannot convert value "True" to type "System.Text.Encoding". Error: "Invalid cast from
'System.Boolean' to 'System.Text.Encoding'.""
    + CategoryInfo          : NotSpecified: (:) [], CimException
    + FullyQualifiedErrorId : MethodArgumentConversionInvalidCastArgument
    + PSComputerName        : localhost