Upgrade .kitchen.yml boxes
Change newrelic-plugin-agent executeable name
Update test-kitchen, berkshelf, kitchen-vagrant versions
Remove ruby cookbook ref in Berksfile, bug was fixed
Upgrade newrelic-plugin-agent automatically
Remove unnecessary platform config values from Test Kitchen config

These are the defaults from Test Kitchen itself; they don't need to be
specified explicitly
Remove default values from Test Kitchen config

No need to specify the Omnibus or Berkshelf, Test Kitchen does it all
right from the get-go
Use Chef Zero instead of Chef Solo for Test Kitchen provisioning
Update version to 0.5.0, update CHANGELOG