Fix frame data records being created for non-ugoira posts.

* Prevent non-ugoira posts from getting dummy frame data records with
  all the columns set to null.
* Make `data` and `content_type` columns NOT NULL, since they should
  always be present.
* Remove the timestamps while we're at it, since they don't seem
  necessary for anything.

db/migrate/20141017231608_fix_pixiv_ugoira_frame_data.rb

@@ -0,0 +1,9 @@
+class FixPixivUgoiraFrameData < ActiveRecord::Migration
+  def change
+    change_table :pixiv_ugoira_frame_data do |t|
+      t.change :data,         :text,   :null => false
+      t.change :content_type, :string, :null => false
+      t.remove_timestamps
+    end
+  end
+end