Ruby versions and improve rakefile

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