docs: load CDN assets over HTTPS

When accessing https://angular-ui.github.io/ui-select/demo-tagging.html, the browser blocks insecure (http://) content.

Author: softwareresearchwork

docs/partials/_header.html

@@ -5,21 +5,21 @@
     <title>AngularJS ui-select</title>
 
     <!--
-      IE8 support, see AngularJS Internet Explorer Compatibility http://docs.angularjs.org/guide/ie
+      IE8 support, see AngularJS Internet Explorer Compatibility https://docs.angularjs.org/guide/ie
       For Firefox 3.6, you will also need to include jQuery and ECMAScript 5 shim
     -->
     <!--[if lt IE 9]>
-      <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.js"></script>
-      <script src="http://cdnjs.cloudflare.com/ajax/libs/es5-shim/2.2.0/es5-shim.js"></script>
+      <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.js"></script>
+      <script src="https://cdnjs.cloudflare.com/ajax/libs/es5-shim/2.2.0/es5-shim.js"></script>
       <script>
         document.createElement('ui-select');
         document.createElement('ui-select-match');
         document.createElement('ui-select-choices');
       </script>
     <![endif]-->
 
-    <script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.5.0/angular.js"></script>
-    <script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.5.0/angular-sanitize.js"></script>
+    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.0/angular.js"></script>
+    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.0/angular-sanitize.js"></script>
 
     <!-- ui-select files -->
     <script src="./dist/select.js"></script>
@@ -28,11 +28,11 @@
     <script src="./assets/demo.js"></script>
 
     <!-- themes -->
-    <link rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.css">
-    <link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/select2/3.4.5/select2.css">    
-    <link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.default.css">
-    <!-- <link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.bootstrap2.css"> -->
-    <!--<link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.bootstrap3.css">--> 
+    <link rel="stylesheet" href="https://netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.css">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/3.4.5/select2.css">    
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.default.css">
+    <!-- <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.bootstrap2.css"> -->
+    <!--<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.8.5/css/selectize.bootstrap3.css">--> 
 
     <style>
         body {