Relax activesupport version lock, add appraisal for test

- Add appraisal
    - for testing a library against different versions of dependencies
    - http://rubygems.org/gems/appraisal
    - https://github.com/thoughtbot/appraisal
- Use generated gemfile in travis-ci against activesupport 3.2, 4.0.0.beta
- Relax activesupport version lock