Update template to be compatible with Ruby 2.0 (Mavericks system ruby)

Since ruby 2.0 is the default system ruby in OS X 10.9 the Boxen module template
should be prepared and tested for ruby 2.0.

- Add CI for Ruby 2.0
  - The tests started to fail with ruby 2.0 as soon as some real work (including
    a class etc.) was done. This was not spotted by Travis because the test were
    not run against ruby 2.0.
- Update gems to newer versions
  - Older puppet versions are not fully compatible with Ruby 2.0
  - Everything else was quite out of date, too