Undocumented dependecy: Powershell >= 3.0

[JeffCarpenter]

Powershell 2.0 returns the following error:

C:\Users\Jeff\Dev\fsharp\websharper.ui.next>build.cmd
Remove-Item : Cannot bind parameter 'ErrorAction'. Cannot convert value "ignore" to type "System.Management.Automation.ActionPreference" due to invalid enumeration values. Specify one of the following enumeration values and try again. The possible enumeration values are "SilentlyContinue, Stop, Continue, Inquire".
At C:\Users\Jeff\Dev\fsharp\websharper.ui.next\tools\restore-packages.ps1:2 char:28
+ rm -r packages -errorAction <<<<  ignore
    + CategoryInfo          : InvalidArgument: (:) [Remove-Item], ParameterBindingException
    + FullyQualifiedErrorId : CannotConvertArgumentNoMessage,Microsoft.PowerShell.Commands.RemoveItemCommand