Address review comments

Author: AndriiMysko

lib/travis/api/v3/models/custom_image.rb

@@ -7,7 +7,7 @@ class Models::CustomImage < Model
     scope :available, -> { where(state: 'available') }
 
     def created_by
-      user_id = custom_image_logs.where(action: 'created').first&.sender_id
+      user_id = custom_image_logs.created.frist&.sender_id
       return unless user_id
 
       User.find(user_id)

lib/travis/api/v3/models/custom_image_log.rb

@@ -2,5 +2,6 @@ module Travis::API::V3
   class Models::CustomImageLog < Model
     after_initialize :readonly!
     belongs_to :custom_image
+    enum action: { created: 'created', used: 'used', deleted: 'deleted', other: 'other' }
   end
 end