Tests fail when run with bundle

Problem:
* Tests failing under `jenkins`
* Failing locally when invoked with `bundle exec`

Investigation results:
```rb
describe foo do
  # it should_be etc.
end
```
was creating different types of classes (`RSpec::ExampleGroups` versus `RSpec::Core::ExampleGroups`) thus returning `nil` for `block` in https://github.com/cargomedia/puppet-packages/blob/master/spec/spec_helper.rb#L15 (different hash key symbols)

I could get it to work either by adapting `spec_helper.rb` to handle both key identifiers or by pinning the required `rspec` gem version. Apparently,  `RSpec::Core::ExampleGroups` is being deprecated from v3.x on.

@tomaszdurka fyi, it's your code

@kris-lab thank you for your support and could you please review?