Update Vagrant stack (and fix broken Gem dependencies)

**Notes:**

- I first started to create very conservative Gem dependency settings (e.g. Vagrant ~1.1, Berkshelf ~1.4, vagrant-berkshelf ~1.1), to fix [travis broken build](https://travis-ci.org/calavera/minitest-chef-handler/builds/11657727). On my way, I finally found better to go forward and make the Vagrant dependency more up to date. Hope you like it...
- I have no experience in rubygems publication and thus I preferred not to bump `gem.version`. But since [https://rubygems.org/gems/minitest-chef-handler](version `1.0.1`) is already published, I guess that if this PR is 
accepted, this value should be changed... (to 1.0.2 or 1.1.0?)

**Changes:**

- 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 tests behave the same for all contributors)
- Two tests were adapted to pass with new basebox
- Skip two failing tests (it looks like these problems should be fixed in sublayers, like Chef resources API or minitest. to be investigated).

Not done yet (no tested yet, and to be discussed):
- Like specs, should we run the Vagrant tests twice (also against Chef
  10?)
- Is an update to minitext 5.0.x straightforward (and recommended)?