Check version of PowerShell in build script

[HebaruSan]

Problem

If you run the current build.ps1 script with PowerShell 2 or earlier, the $PSScriptRoot variable isn't set, which means several of the build script's variables are set to either the empty string or incomplete values. This has many consequences, for example the _build directory is created in the root of the drive.

CKAN/build.ps1

Lines 12 to 18 in 700f709

	$RootDir = "${PSScriptRoot}"
	$ScriptFile = "${RootDir}/build.cake"
	$BuildDir = "${RootDir}/_build"
	$ToolsDir = "${BuildDir}/tools"
	$PackagesDir = "${BuildDir}/lib/nuget"
	$NugetExe = "${ToolsDir}/NuGet/${NugetVersion}/nuget.exe"
	$PackagesConfigFile = "${RootDir}/packages.config"

Changes

This pull request checks the currently running version of PowerShell and prints an error if it's less than 3.0:

We could try to make the script backwards compatible with PowerShell 2.0, but PowerShell is up to 5.1 now, so it's probably better just to upgrade.

I wanted to use Write-Error, but apparently there's no way to turn off its (in this case useless) stack trace output.