Use appraisal instead of handmaid(?) gemfiles

I can test easily against AR3.1, AR3.2, AR4.0.0.beta, Sinatra and any gemfile with appraisal.
Replace five gemfiles in ```.travis.yml``` (active_record_32, data_mapper_12, mongo_mapper, mongoid_30, sinatra)
Run tests at once.
```ruby
bundle exec rake appraisal:install
bundle exec rake appraisal spec
```

If run with specific gemfile:
```ruby
bundle exec rake appraisal:sinatra spec
```

https://rubygems.org/gems/appraisal
https://github.com/thoughtbot/appraisal