Use absolute URL for OG and Twitter images (vuejs#545)

Author: phanan

themes/vue/layout/layout.ejs

@@ -10,12 +10,12 @@
         <meta property="og:type" content="article">
         <meta property="og:title" content="<%- page.title ? page.title + ' - ' : '' %>vue.js">
         <meta property="og:description" content="<%- theme.site_description %>">
-        <meta property="og:image" content="/images/logo.png">
+        <meta property="og:image" content="https://<%- theme.root_domain %>/images/logo.png">
 
         <meta name="twitter:card" content="summary">
         <meta name="twitter:title" content="<%- page.title ? page.title + ' - ' : '' %>vue.js">
         <meta name="twitter:description" content="<%- theme.site_description %>">
-        <meta name="twitter:image" content="/images/logo.png">
+        <meta name="twitter:image" content="https://<%- theme.root_domain %>/images/logo.png">
 
         <link href='//fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,600|Roboto Mono' rel='stylesheet' type='text/css'>
         <link href='//fonts.googleapis.com/css?family=Dosis:500&text=Vue.js' rel='stylesheet' type='text/css'>