Enable CI for CentOS6

[arm4b]

Implements #69

Enable CI for CentOS6 (kitchen-docker in TravisCI).

Additionally, we run the task with allow_failures in Travis until CentOS6 is fully supported by ansible-st2 playbooks as part of https://github.com/StackStorm/ansible-st2/milestone/2 plan.

[arm4b]

This works, Kitchen could SSH to CentOS6, install Ansible and run playbooks.
Playbooks inside the CentOS6 Docker container are failing because our playbooks doesn't support EL yet.
But this will help during the development process to see the build logs for every PR.

[humblearner]

👍