Use an absolute URL for social images

Author: colinodell

docs/_layouts/default.html

@@ -38,7 +38,7 @@
     {% else %}
         <meta property="og:description" content="{{ site.data.project.description }}" />
     {% endif %}
-    <meta property="og:image" content="/images/commonmark-social.png" />
+    <meta property="og:image" content="https://commonmark.thephpleague.com/images/commonmark-social.png" />
     <link rel="stylesheet" href="//theme.thephpleague.com/css/all.css?{{ site.github.build_revision }}">
     <link rel="stylesheet" href="/custom.css?{{ site.github.build_revision }}">
     <link href="//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">

docs/_layouts/homepage.html

@@ -16,7 +16,7 @@
     <meta property="og:url" content="https://commonmark.thephpleague.com/" />
     <meta property="og:title" content="{{ site.data.project.site_title }} - {{ site.data.project.tagline }}" />
     <meta property="og:description" content="{{ site.data.project.description }}" />
-    <meta property="og:image" content="/images/commonmark-social.png" />
+    <meta property="og:image" content="https://commonmark.thephpleague.com/images/commonmark-social.png" />
     <link rel="stylesheet" href="//theme.thephpleague.com/css/all.css?{{ site.github.build_revision }}">
     <link rel="stylesheet" href="/homepage.css?{{ site.github.build_revision }}">
     <script async src="https://www.googletagmanager.com/gtag/js?id={{  site.data.project.google_analytics_tracking_id }}"></script>