Merge branch 'fix/irreversible-migration' into 'master'

Fix undefined method when reverting migration `RemoveProjectsPushesSinceGc`

See merge request !6375

db/migrate/20160913162434_remove_projects_pushes_since_gc.rb

@@ -14,6 +14,6 @@ class RemoveProjectsPushesSinceGc < ActiveRecord::Migration
   end
 
   def down
-    add_column_with_default! :projects, :pushes_since_gc, :integer, default: 0
+    add_column_with_default :projects, :pushes_since_gc, :integer, default: 0
   end
 end