Use protocol-relative avatar URLs

app/decorators/user_decorator.rb

@@ -23,7 +23,7 @@ class UserDecorator < ApplicationDecorator
 
   def gravatar_url
     hash = Digest::MD5.hexdigest(model.email.to_s.downcase)
-    "http://gravatar.com/avatar/#{hash}?s=64"
+    "//gravatar.com/avatar/#{hash}?s=64"
   end
 
 end

app/views/layouts/application.html.slim

@@ -42,4 +42,4 @@ html[lang="en"]
         p.title
           ' Developed with passion by
           = link_to 'Marcin Kulik', 'https://github.com/sickill'
-          img.avatar alt="Marcin" src="http://gravatar.com/avatar/9cecfc695240b56e5d3c1a5dc3830967"
+          img.avatar alt="Marcin" src="//gravatar.com/avatar/9cecfc695240b56e5d3c1a5dc3830967"