require puppetlabs_spec_helper/rake_tasks

i think that's all you need with a recent version of rspec-puppet and
puppetlabs_spec_helper install.
changed include_class to contain_class, added concat_basedir

- in newer version of rspec-puppet include_class changed to contain_class,
  this fixes an deprecation warning.

- puppetlabs-concat (former ripienaar-concat) require a fact
  concat_basedir that comes with the module.