Make cipd bootstrap script compatible with PowerShell 2.0

BUG=673940

Change-Id: I64d26454db3693499d4fb8f82bafb9eb04125c0d
Reviewed-on: https://chromium-review.googlesource.com/420408Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

cipd.ps1

@@ -12,7 +12,7 @@ function GetEnvVar([string] $key, [scriptblock] $defaultFn) {
 }
 
 $cipdClientVer = GetEnvVar "CIPD_CLIENT_VER" {
-  Get-Content (Join-Path $myPath -ChildPath 'cipd_client_version') -First 1
+  Get-Content (Join-Path $myPath -ChildPath 'cipd_client_version') -TotalCount 1
 }
 $cipdClientSrv = GetEnvVar "CIPD_CLIENT_SRV" {
   "https://chrome-infra-packages.appspot.com"