The Gemfile.lock should be versionned

This allow homogeneity between all the dev. Otherwise we won't be sure
everybody got the exact same gem version.
Bump the version of test-kitchen
Clean the ubuntu .kitchen.yml details
Add a debian vm in the .kitchen.yml
Add a test to ensure smtpd/smtpctl is in the PATH
Add a "remove_exim4" recipe

This reciepe is use during test with the debian VM. We need to remove
exim4 before testing OpenSMTPD, otherwise the test will not pass.
Add specific debian files (init and default)
Move upstart template to an ubuntu folder
Clean the init script creation part

* Better at the end
* Thus no need to reload the configuration
Add debian as a supported OS
Make the recipe debian compatible
Update README with Ubuntu support
Some foodcritic and rubocop correction