Use 1200x1207 image for og:image

Because
https://developers.facebook.com/docs/sharing/best-practices#images says
"Use images that are at least 1200 x 630 pixels for the best display on
high resolution devices."

_layouts/skeleton.html

@@ -14,7 +14,7 @@
   <meta property="fb:page_id" content="201193596592346"><!-- groonga -->
   <meta property="fb:admins" content="664204556"><!-- kouhei.sutou -->
   <meta property="og:type" content="website">
-  <meta property="og:image" content="{{ site.url }}/images/logos/groonga-icon-full-size.png">
+  <meta property="og:image" content="{{ site.url }}/images/groonga-icon.png">
   <meta property="og:site_name" content="Groonga">
   <meta property="og:url" content="{{ site.url }}{{ page.url | remove:'index.html' }}">
   {% if page.description %}<meta property="og:description" content="{{ page.description }}">{% endif %}