Fix Linting

[cocker-cc]

Fix Linting according to best Practices stated in PDK-Templates.

PDK and the PDK-Templates themselves cannot currently be updated,
because the current Rspec-Tests together with the custom spec_helper.rb
cannot be used with recent PDK-Templates.

[cocker-cc]

Rspec fails with, f.e.…

Evaluation Error: Error while evaluating a Function Call, 
undefined method `negative?' for -1:Fixnum 
(file: /home/circleci/project/spec/fixtures/modules/datadog_agent/manifests/params.pp, line: 22, column: 37) 
on node ip-10-0-89-162.ec2.internal

… because stdlib's Function load_module_metadata() uses the Ruby-Method numeric#negative?, which is available from Ruby 2.3.0 only. Ruby versions older than 2.3 are deprecated in Puppet 5.5.1 and will be removed in Puppet 6..

This means: Puppet-Versions older than 5.5.1 cannot be tested and supported anymore with this PR.

I let .fixtures.yml pull the Modules' most recent Versions, because this is, what Endusers potentially (and most probably) are using – so the Rspec-Tests should test against these most recent Versions!