I think it should match the version of Puppet the to-be-released version of the spec_helper is designed to operate with.  For example, if the next release of the spec_helper is designed to work with Puppet 3, then this should be `"rspec", "~> 2.11.0"` matching https://github.com/puppetlabs/puppet/blob/3.6.1/Gemfile#L36