Figured out what was wrong 
I didn’t customise/configure the Travis testing, so it seems to have prevented the plugin from working.
So I tried removing everything except the plugin.rb file and it started working for me. I modified the logic slightly for my use case also.
As it is currently, TL0+ can message an admin if their trust level falls below the ‘min trust to send messages’ configured, but they cannot message a moderator.
So to modify this, this section needs to be changed very slightly (appreciate the clear comments 
):
# Have to be a basic level at least -- and now: OR SENDING TO ADMIN
(is_group || @user.has_trust_level?(SiteSetting.min_trust_to_send_messages) || notify_moderators || target.admin) &&
If you want to allow messaging to any staff member (who has not specifically disabled receiving personal messages), then change it to:
# Have to be a basic level at least -- and now: OR SENDING TO STAFF
(is_group || @user.has_trust_level?(SiteSetting.min_trust_to_send_messages) || notify_moderators || target.staff?) &&
If you want to enable messaging to mods only and not admins:
# Have to be a basic level at least -- and now: OR SENDING TO MODERATOR
(is_group || @user.has_trust_level?(SiteSetting.min_trust_to_send_messages) || notify_moderators || target.moderator) &&
