update cdn (#539)

Author: Arhell

_layouts/bootstrap.html

@@ -14,7 +14,7 @@
     <!-- CSS -->
     <link href="https://fonts.googleapis.com/css2?family=Roboto+Mono:wght@400;500;700&display=swap" rel="stylesheet">
     <link href="https://fonts.googleapis.com/css?family=Lato&display=swap" rel="stylesheet">
-    <link href="//netdna.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css" rel="stylesheet">
+    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css" integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu" crossorigin="anonymous">
 
     <link rel="stylesheet" href="{{ '/css/style.css' | relative_url }}">