Migration can not be rolled back

[mebibou]

The 49 so far migrations can not be rollbacked, I keep getting errors on many migrations:

• 20160627055749_remove_storytime_role_id_from_users.storytime.rb
• 20160627055737_remove_homepage_path_from_storytime_sites.storytime.rb
• 20160627055731_remove_root_page_content_from_storytime_sites.storytime.rb
• ...

Please make sure that every migration can be rolled back

[bcroesch]

If you're interested in opening a pull for this, we're happy to review & merge.