Update the CI to test against more recent Ansible/Python/OS versions

[atosatto]

This PR implements multiple enhancements to the versions of the software used by the Travis CI infrastructure:

• add Python 3 to the tests matrix
• add Ansible 2.8 and Ansible 2.9 to the tests matrix
• remove Ansible 2.5 and Ansible 2.6 from the tests matrix (as these are unmaintained and eol)
• add CentOS 8 as test target
• update the testinfra code to test against the 4.2.0 release of the PowerDNS Recursor
• speedup the tests execution enabling Ansible cache and pipelining

[atosatto]

I had to disable CentOS 8 due to ansible/ansible#64963. :(