Commit messages should be validated/linted on Travis

[joakimbeng]

Background:

Currently commit messages are only linted by a git hook, which requires cloning the repository and running npm install for the hook to be installed properly.

This makes it possible to push commits that doesn't follow the Conventional Commits specification. Which will break the automatic versioning and releases.

The solution:

Add @commitlint/travis-cli as a dependency and run it in .travis.yml.