Nerf Rival Back-Compat Adapter

This is a mash up of @guohc's [Nerf rail adapter](https://www.thingiverse.com/thing:2150989) and @kaiyote's [Nerf Rival Tactical Rail](https://www.thingiverse.com/thing:1645341). My son accidentally bought a Nerf Rival accessory, but due to new Rival rail type it doesn't fit his Nerf Elite. This adapter now allows him to easily slap his Rival accessories on whatever other Nerf guns he has in his arsenal.

Updated 2017-08-04:
- Added a "nubbin" on the older tactical rail so that the rail doesn't slide off.
- Made the Rival side of the rail about 1mm wider to get a better fit for attachments.
- Added a "no overhang" version, if your Rival attachment doesn't need the gaps.