Fix for dnvm update-self on PowerShell 2.0

PSScriptRoot variable is not available in PowerShell 2.0. This change make use of a Get-ScriptRoot function which return current script location using PSScriptRoot (in PS >= 3.0) or MyInvocation.MyCommand.Path (in PS < 3.0)

Fix for:
#391

src/dnvm.ps1

@@ -846,6 +846,14 @@ function Is-Elevated() {
     return $user.IsInRole([Security.Principal.WindowsBuiltInRole] "Administrator")
 }
 
+function Get-ScriptRoot() {
+    if (!$PSVersionTable.PSVersion.Major -ge 3) {
+        return $PSScriptRoot
+    }
+
+    return Split-Path $script:MyInvocation.MyCommand.Path -Parent
+}
+
 ### Commands
 
 <#
@@ -862,10 +870,11 @@ function dnvm-update-self {
     _WriteOut "Updating $CommandName from $DNVMUpgradeUrl"
     $wc = New-Object System.Net.WebClient
     Apply-Proxy $wc -Proxy:$Proxy
-
-    $dnvmFile = Join-Path $PSScriptRoot "dnvm.ps1"
-    $tempDnvmFile = Join-Path $PSScriptRoot "temp"
-    $backupFilePath = Join-Path $PSSCriptRoot "dnvm.ps1.bak"
+	
+	$CurrentScriptRoot = Get-ScriptRoot
+    $dnvmFile = Join-Path $CurrentScriptRoot "dnvm.ps1"
+    $tempDnvmFile = Join-Path $CurrentScriptRoot "temp"
+    $backupFilePath = Join-Path $CurrentScriptRoot "dnvm.ps1.bak"
 
     $wc.DownloadFile($DNVMUpgradeUrl, $tempDnvmFile)