$w = Get-SPWeb http://acme.com
$uca = $w.UserCustomActions.Add()
$uca.Name = "jQuery CDN"
$uca.Location = "ScriptLink"
$uca.ScriptBlock = "document.write('<script type=`"text/javascript`" src=`"//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js`"></' + 'script>');"
$uca.Sequence = 111
$uca.Update()