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
- removed gemfiles/* from version control
- set travis to run the appraisal task over each db