Get rid of the deprecated appraisal rake tasks in favor of running
appraisal directly. To run the whole suite, we can run
`bundle exec appraisal rake`.
I also went ahead and updated all the gemfiles.
- Add Rails 5.1 and 5.2 to Appraisal
- Avoid running combinations we know are going to fail by moving to
"exclude" instead of "allow_failures"
- Use updated migration class for Rails 5.1+