change social image to .png

//cc @nqst

website/themes/uppy/layout/layout.ejs

@@ -1,7 +1,7 @@
 <% var isIndex = page.path === 'index.html' %>
 <% var title = page.title ? page.title + ' — ' + config.title : config.title %>
 <% var excerpt = page.excerpt ? page.excerpt.replace(/(<([^>]+)>)/ig, '').substring(0, 400) : config.description %>
-<% var image = page.image ? page.image : 'http://uppy.io/images/uppy-social-pink.jpg' %>
+<% var image = page.image ? page.image : 'http://uppy.io/images/uppy-social-pink.png' %>
 <% 
 if (page.series) {
   title = page.series + ': ' + page.title