ci(travis): use default matrix after `centos-6` image fix

[myii]

• Automated using https://github.com/myii/ssf-formula/pull/105
• https://github.com/netmanagers/salt-image-builder/issues/18

[saltstack-formulas-travis]

:tada: This PR is included in version 0.5.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: