Update to the newest inherited_resources

The 1.5.0 version of inherited_resources was causing issues with belongs_to associations in a controller.  See the discussion here - https://github.com/activeadmin/activeadmin/pull/3193

Version 1.5.1 was released to fix this issue.  This PR updates AA to require version 1.5.1 or higher of inherited_resources.
Why not '~> 1.4', '!= 1.5.0', or is there a reason to cat of 1.4?