style: update Bootstrap to v4.3.1

Author: trollixx

_includes/head.html

@@ -2,9 +2,9 @@
 <title>{% if page.title %}{{ page.title }} · {% endif %}{{ site.name }}{% if page.slug == "home" %} - {{ site.description }}{% endif %}</title>
 <meta http-equiv="X-UA-Compatible" content="IE=edge">
 <meta name="viewport" content="width=device-width, initial-scale=1">
-<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
-<link href="{{ site.baseurl }}/static/css/styles.css" rel="stylesheet">
-<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" integrity="sha384-wvfXpqpZZVQGK6TAh5PVlGOfQNHSoD2xbE+QkPxCAFlNEevoEH3Sl0sibVcOQVnN" crossorigin="anonymous">
+<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="{{ site.baseurl }}/static/css/styles.css">
+<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" integrity="sha384-wvfXpqpZZVQGK6TAh5PVlGOfQNHSoD2xbE+QkPxCAFlNEevoEH3Sl0sibVcOQVnN" crossorigin="anonymous">
 <link rel="icon" href="/static/img/favicon-16.png" sizes="16x16" type="image/png">
 <link rel="icon" href="/static/img/favicon-32.png" sizes="32x32" type="image/png">
 <link rel="icon" href="/static/img/favicon-48.png" sizes="48x48" type="image/png">