[wip] Use appraisal

I can test easily against AR3.1, AR3.2, AR4.0.0.beta, Sinatra and any gemfile with appraisal.

Run tests at once.
```ruby
bundle exec rake appraisal:install
bundle exec rake appraisal spec
```
This pull request is work in progress, and only AR3.2 yet.

https://rubygems.org/gems/appraisal
https://github.com/thoughtbot/appraisal