release v2.3.0 (2015-08-30)

• Update chef links to use chef.io domain.
• Update contact information and links after migration.
• metadata: Add source_url and issues_url.
• Tests:
  • Move ChefSpec tests to test/unit.
  • Rakefile: Add clean task.
  • Travis CI: Use kitchen-in-travis version 0.3.0.
  • Gemfile: Update RuboCop to 0.33.0.
• Documentation:
  • README: Improve description.