ci(travis): merge `rubocop` linter into main `lint` job

[myii]

• Semi-automated using https://github.com/myii/ssf-formula/pull/65

[myii]

Self-merging, since no impact on the functionality of this formula.

[saltstack-formulas-travis]

:tada: This PR is included in version 0.22.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: