MotionKit and ProMotion fixes

1. Use `master` (`pm2` has been abandoned) or `"~> 2.0.0.rc3"`
2. Use `style:` not `styles:` (sorry about that -- error in our earlier version of `pm2`)
3. Set accessor methods on your cell (see the diff)
4. Set your wrapper view (`:entry_cell`) constraints to avoid the 20 pt issue

Ref: https://github.com/clearsightstudio/ProMotion/issues/488