Make appraisal task easier to run; run it under travis

- `rake spec` now implicitly copies over spec/database.yml.example if necessary
- `rake appraise` now removes existing appraisal-generated gemfiles, installs new ones based on Appraisal, and then runs the appraisal task
- regenerated the appraisal gemfiles
- set travis to run across the generated gemfiles

I've confirmed I can run tests as
- `wwtd` # [what would travis do](https://github.com/grosser/wwtd)
- `rake appraise` # run all [appraisal](https://github.com/thoughtbot/appraisal) tasks
- `rake`