Run / Check Travis after merging.

Currently Travis builds are executed only when a Pull Request is created. This does not cover fully situations when a PR is created to a stable branch and later merged up to master. Sometimes conflicts need to be resolved when merging up so the risk of introducing regressions due to the human factor is severe.
Ideally Travis builds should be executed on the base branch from PR and all branches up to master. This way the risk related to resolving conflicts would be minimised.

TODO:

• introduce for ezplatform-page-builder - done
• decide on range / scope of bundles
  Proposed:
• kernel (all branches)