Added versions to 'add_development_dependency' calls to reduce gem build warnings

When running `gem build [app_name].gemspec` on a GLI project, the following warnings are thrown:

    WARNING:  open-ended dependency on rake (>= 0, development) is not recommended
      if rake is semantically versioned, use:
        add_development_dependency 'rake', '~> 0'
    WARNING:  open-ended dependency on rdoc (>= 0, development) is not recommended
      if rdoc is semantically versioned, use:
        add_development_dependency 'rdoc', '~> 0'
    WARNING:  open-ended dependency on aruba (>= 0, development) is not recommended
      if aruba is semantically versioned, use:
        add_development_dependency 'aruba', '~> 0'

This change simply adds the recommended version strings into the scaffold.rb file to eliminate the warnings.