Add html5ever repo support to homu.

[metajack]

To enable repos you have to:

1. Add them in the homu/cfg.toml by adding to the list as ('$owner', '$repo')
2. Add a webhook for http://build.servo.org:54856/github to the repo webhooks for push,pull request, and issue comments with the secret key (see private data pillar for the key)
3. Add a webhook in .travis.yml in the repo pointing at http://build.servo.org:54856/travis

Additionally you may need to reset the travis token and associated user in the travis service in the github admin panel for the repo. (use bors-servo as the user).

[metajack]

r? @edunham

[edunham]

If you've already set the value for reviewers in the pillar, I think it should be good. I often trust syntax highlighting to point out errors in files like this, but github's clearly unaware that it's actually jinja-templated. html5ever already has the hook that you described (https://github.com/servo/html5ever/blob/master/.travis.yml). Will you do the repo webhooks, or would you like me to?

[metajack]

There's no reviewers value in the pillar. The value is initially set here in the template.
I already did the mentioned steps for html5ever, but we'll have to repeat those steps for the other repos we enable. i wasn't sure where to put such documentation, so I figured I'd at least get it into the PR description.

Suggestions on where docs should go?

[edunham]

How does https://github.com/servo/saltfs/wiki/Adding-a-repo-to-Homu look for now, and we can replace it with a link to the new location if we find a better place?

I'll also update them if the process doesn't work right for this PR.

[metajack]

Excellent!

[metajack]

Any other concerns, or is this ready to merge and deploy?

[edunham]

If there are any syntax errors, I'm not seeing them. Let's merge and deploy 👍