Add travis label

[braybaut]

Description

This is a few lines to add the Travis CI label to the README.md

Checklist:

I have:

• checked my changes follow the style of the existing code / OpenFaaS repos
• updated the documentation and/or roadmap in README.md
• read the CONTRIBUTION guide
• signed-off my commits with git commit -s
• added unit tests

[derek]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide.
Tip: if you only have one commit so far then run: git commit --amend --sign-off and then git push --force.

[alexellis]

LGTM, thank you!