Fix migration to make it fast

Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20160721081015_nullify_has_external_wiki_in_projects.rb → db/migrate/20160721081015_drop_and_readd_has_external_wiki_in_projects.rb

-class NullifyHasExternalWikiInProjects < ActiveRecord::Migration
+class DropAndReaddHasExternalWikiInProjects < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
   def up
-    execute("UPDATE projects SET has_external_wiki = NULL")
+    remove_column :projects, :has_external_wiki, :boolean
+    add_column :projects, :has_external_wiki, :boolean
   end
 
   def down