Reduce the number of default assumptions

* Add CentOS to kitchen platforms to match default `kitchen init`
  behavior in test-kitchen 1.0
* Move apt recipe to ubuntu platform run list
* Remove `supports` from metadata
* Make the version sub-1.0, to accomodate development versions under
  semver
* Remove the package resource in the default recipe, and the related
  specs
* Use generalized default config in the spec_helper that is commented
  by default
Fix spec_helper to work with serverspec 0.13

See https://github.com/serverspec/serverspec/commit/797b14015d5a881f5fea531e2df44db672fc1e08#commitcomment-4760150
serverspec does not use RSpec.configuration.os anymore

See https://github.com/serverspec/serverspec/commit/797b14015d5a881f5fea531e2df44db672fc1e08#commitcomment-4760496
Reducing assumptions redux, feedback

* Remove apt
* Fix version in changelog
* Use source in berksfile (we can assume because Gemfile locks the
  version to < 3.0)
* Remove vi assumption, it's not used elsewhere
Merge remote-tracking branch 'jtimberman/reduce-assumptions' into reduce-assumptions
Fix Berksfile
Remove Gemfile.lock
Update documentation of available Rake tasks
Tweak style of metadata.rb
Remove superfluous comment from default recipe
Fix ChefSpec spec helper
ServerSpec -> Serverspec
Merge branch 'reduce-assumptions'
Clean up .gitignore
Remove Vagrantfile
Link to ChefSpec README on GitHub
Tweak Contributing guidelines
Log message and test for it as suggested by @jtimberman
Add rubocop to Gemfile
Add rubocop Rake task