Bumped version to 1.5.0 for release

* BREAKING CHANGE: The gem list in bootstrap['gems'] must be converted
  from a list of Arrays to a Hash in yaml definition files, please
  look at README.md
* Added functionality to apply specific named configs in a group
  (merges into and takes precedence over the group config) for nodes
  within a group that have unique attributes
* Improved Rivet's CLI option parser. It now uses OpenStruct instead
  of a Hash for the options, has cleaner user error messages with cmd
  help displayed and supports exit status codes
* Updated README.md and CHANGELOG.md