Use gemfile generated by appraisal instead of handmaid gemfiles

Use generated gemfile instead of handmaid gemfile

This is prepared by appraisal
https://rubygems.org/gems/appraisal
https://github.com/thoughtbot/appraisal

Test default:
```
$ bundle exec rake
```

Test with appraisal:
```
$ bundle exec rake appraisal:install
#=> generate gemfiles/*gemfile from Apraisals file, and bundle each files
$ bundle exec rake appraisal
# => run test against each gemfiles
$ bundle exec rake appraisal:rails4_0
# => run test against gemfiles/rails4_0.gemfile
```