PowerShell script uses .NET 4 features, so setting minimal Powershell version

The Powershell script code uses String.IsNullOrWhiteSpace. This method was introduced in .NET 4.0. PowerShell 3.0 is required to use .NET 4.0 features.
Merge pull request #1 from ringods/master

PowerShell script uses .NET 4 features, so setting minimal Powershell version