Integrate CheckStyle checks in PR build

[ledoyen]

Today, when submitting a PR, there is no community feedback if proposed code does not comply with checkstyle rules.

Only the core team can detect such an oversight and provides a feedback to external commiters.
It is time consuming for the core team (I assume 😄), and does not avoid code containing new violations from being merged.

A quick action could be to integrate a Travis build running gradle check to PR workflow.

[snicoll]

@ledoyen thanks for the feedback, we intend to move to concourse and add a pipeline to build pull requests.

[snicoll]

Superseded by #22490