FIX: Missing letter avatars

cc @zogstrip

It seems that the version string was ammended to return the ImageMagick
version. This caused the guard in the user avatars method to fail as the
versions weren't the same.

I changed it so it is comparing the right version, but I wonder if this
is bad as the controller is no longer using the ImageMagick version. Can
you please review?

app/controllers/user_avatars_controller.rb

@@ -24,7 +24,7 @@ class UserAvatarsController < ApplicationController
     params.require(:version)
     params.require(:size)
 
-    return render_dot if params[:version] != LetterAvatar.version
+    return render_dot if params[:version].to_i != LetterAvatar::VERSION
 
     image = LetterAvatar.generate(params[:username].to_s, params[:size].to_i)