Update Vagrant stack

- Gemfile and minitest-chef-handler.gemspec are now more restrictive
  about dependency versions
- Use latest Vagrant version (1.3.3)
- Vagrantfile is now in configuration format "2" (only compatible with
  Vagrant 1.1+)
- Vagrantfile refers to an explicit basebox (so that contributors can
  verify with the same machine)
- Two tests adapted to work with new basebox
- Skip two failing tests

Not done yet (to be discussed):
- Like specs, should we run the Vagrant tests twice (also against Chef
  10?)
- Update to minitext 5.0.x?
Merge pull request #68 from gildegoma/update-vagrant-stack

Update Vagrant stack (and fix broken Gem dependencies)