make migration reversible

db/migrate/20150417000045_cleanup_the_build_model.rb

 class CleanupTheBuildModel < ActiveRecord::Migration
   def change
-    remove_column :builds, :push_data
-    remove_column :builds, :before_sha
-    remove_column :builds, :ref
-    remove_column :builds, :sha
-    remove_column :builds, :tmp_file
+    remove_column :builds, :push_data, :text
+    remove_column :builds, :before_sha, :string
+    remove_column :builds, :ref, :string
+    remove_column :builds, :sha, :string
+    remove_column :builds, :tmp_file, :string
   end
 end