Add CentOS6 CI in Travis

[arm4b]

We started adding EL support in #65 and #65 as part of #5.
Working without CI is bad for quality, testing and reviewing.

There is a need to add CentOS6 integration Testing in Travis as we already do for Ubuntu Trusty and Ubuntu Xenial. The framework is already setup.

It's possible to make CentOS6 build non-voting to skip the failure for entire build (see travis.yml example for st2 repo) until we re-write all roles to support new platforms.

See:

• https://travis-ci.org/StackStorm/ansible-st2
• https://github.com/StackStorm/ansible-st2/blob/master/.travis.yml
• https://github.com/StackStorm/ansible-st2/blob/master/.kitchen.yml

cc @humblearner