Default to Chef ~> 11.0 for development

Bundler seemed to use Chef v11.2.0 for some reason with the old
requirement.
Add guard-rspec setup
Move berkshelf installation to spec_helper

Copy the idea from "yum" cookbook.
Reorganize Gemfile

Remove :test group as we always want to install those gems.
Add test-kitchen gems to Gemfile

Reminder: `gem uninstall vagrant -a`
Update changelog