[Meta] Run gitlint in Travis

[sigmavirus24]

It would be nice to enforce something of a commit message style automatically. I refrain from reviewing those mostly because I know it's not convenient.

@omgjlk I suspect you have the most opinions on this.

Docs:
https://jorisroovers.github.io/gitlint/#using-gitlint-in-a-ci-environment

[omgjlk]

I do like nice commit messages. It's worth turning this on to experiment. Trying it for a while and then shutting it off if it gets too annoying to work with.