Changed Migration: InnoDB can't store Indexes over 767 bytes. UTF-8 on MySQL uses 3 byte per char so the max would be: 255 Chars.

The index is not really needed, so I removed it.

db/migrate/20141126000001_create_avatar.rb

@@ -18,7 +18,6 @@ class CreateAvatar < ActiveRecord::Migration
     add_index :avatars, [:o_id, :object_lookup_id]
     add_index :avatars, [:store_hash]
     add_index :avatars, [:source]
-    add_index :avatars, [:source_url]
     add_index :avatars, [:default]
   end