remove avatars from gravatar.com

regarding privacy this service is absolutely terrible
http://meta.stackexchange.com/questions/44717/is-gravatar-a-privacy-risk

rails/app/helpers/application_helper.rb

 module ApplicationHelper
 
     def avatar_url(email)
-        default_url = "https://allgo.inria.fr/images/guest.png"
-        gravatar_id = Digest::MD5.hexdigest(email.downcase)
-        "https://gravatar.com/avatar/#{gravatar_id}.png?s=48&d=#{CGI.escape(default_url)}"
+        "https://allgo.inria.fr/images/guest.png"
     end
 
     def appicon_url(app)