.ruby-gemset: Remove unnecessary file.

`bundle exec` takes care of partitioning the gems for us.
.ruby-version: Make compatible with rbenv.

The spec for this file does not include the "ruby-" prefix nor the
gem set name.  Rbenv will complain if they are here.
.gitignore: Lock files should be committed.
Gemfile: Reorganize dependencies into groups.
Vagrantfile: Use bento-based boxes.
spec tests: Move cookbook vendoring into spec_helper.
integration tests: Replace minitest with BATS.
Merge branch 'feature/replace-minitest-with-bats' into develop