make migrations reversible

db/migrate/20151019111551_fix_build_tags.rb

 class FixBuildTags < ActiveRecord::Migration
-  def change
+  def up
     execute("UPDATE taggings SET taggable_type='CommitStatus' WHERE taggable_type='Ci::Build'")
   end
+
+  def down
+    execute("UPDATE taggings SET taggable_type='Ci::Build' WHERE taggable_type='CommitStatus'")
+  end
 end

db/migrate/20151019111703_fail_build_without_names.rb

 class FailBuildWithoutNames < ActiveRecord::Migration
-  def change
+  def up
     execute("UPDATE ci_builds SET status='failed' WHERE name IS NULL AND status='pending'")
   end
+
+  def down
+  end
 end

db/migrate/20151023112551_fail_build_with_empty_name.rb

 class FailBuildWithEmptyName < ActiveRecord::Migration
-  def change
+  def up
     execute("UPDATE ci_builds SET status='failed' WHERE (name IS NULL OR name='') AND status='pending'")
   end
+
+  def down
+  end
 end