chore(travis): add commitplease validation to ci-checks

This will provide feedback to contributors without getting in the way of writing invalid commit
messages locally.

The git hook integration is turned off.

Committers who push directly to the repo can be expected to use correct commit messages ;)
Most changes go through PRs anyway.

Note that "Merge commit" messages and everything starting with "WIP" is always allowed
by commitplease. Follow issue https://github.com/jzaefferer/commitplease/issues/101 for more info.

Related to #14888
Closes #16097