Change ruby versions

Curently it's not possible to compile some versions of ruby >2.0 on
Ubuntu 14.04, so change 2.0.0-p451 to p498 and change 2.1.0 to 2.1.5.
Add default task for rake command

The command "bundle exec rake" must be run checkstyle and chefspec
tests.
With this, no need to add script part into .travis.yml file.
Remove again 14.04

After discussion with Sliim, add 2.1.5 but remove 14.04 support in
kitchen.yml file.
Merge pull request #3 from Numergy/ruby-versions

Ruby versions and improve rakefile