fixed loading of jQuery when site loaded using SSL

bibtexbrowser.php

@@ -2474,7 +2474,7 @@ function poweredby() {
 function javascript() {
   // we use jquery with the official content delivery URLs
   // Microsoft and Google also provide jquery with their content delivery networks
-?><script type="text/javascript" src="http://code.jquery.com/jquery-1.5.1.min.js"></script>
+?><script type="text/javascript" src="//code.jquery.com/jquery-1.5.1.min.js"></script>
 <script type="text/javascript" ><!--
 // Javascript progressive enhancement for bibtexbrowser
 $('a.biburl').each(function() { // for each url "[bibtex]"