change dep from ripienaar/concat -> puppetlabs/concat

This is to prevent module namespace conflicts with other modules that
largely switched to using puppetlabs/concat.
update Gemfile source

To remove this warning:

    The source :rubygems is deprecated because HTTP requests are insecure.
    Please change your source to 'https://rubygems.org' if possible, or
    'http://rubygems.org' if not.
change rspec include_class matcher to contain_class

To suppress this warning with rspec-puppet >=  1.0.0

    DEPRECATION: include_class is deprecated. Use contain_class instead.
fix: added fail message: darwin not supported
s/include_class/contain_class/ and updated travis.yml
Merge pull request #28 from jhoblitt/feature/concat_dep

Feature/concat dep
Updated tests for darwin.
Merge github.com:haraldsk/puppet-module-nfs