Set up PR Preview and Travis deploy to gh-pages

[foolip]

PR Preview documentation:
https://github.com/tobie/pr-preview#configuration-file

Travis setup based on setup in https://github.com/w3c/speech-api.

[foolip]

@atotic I set this up after noticing that #56 still isn't reflected in https://wicg.github.io/ResizeObserver/. There's also a bit of Travis-side setup you need to do, I can guide you on chat if you haven't done it before.

[foolip]

@lukebjerring FYI, if you still see the semicolon missing in reffy-reports, this is what would fix it.

[atotic]

looks good. Would love to get some instructions on how am I supposed to use this. I usually run bikeshed locally, and upload changes manually. Making this automatic would be useful.

[foolip]

What will happen is that on every push to master, a VM (on Travis) will be booted up and run the steps in .travis.yml, bikeshed spec being the most interesting step. The result (index.html) will be committed and pushed to the gh-pages branch, facilitated by https://docs.travis-ci.com/user/deployment/pages/.

[foolip]

Alright, I've configured https://travis-ci.org/WICG/ResizeObserver in a way that I think will work. I'll merge this PR and watch the build and send follow-ups if it still doesn't work.

[foolip]

https://travis-ci.org/WICG/ResizeObserver/builds/413276563 ran successfully and pushed the result to 63967a7. The result is visible at https://wicg.github.io/ResizeObserver/#resize-observer-callback and includes the semicolon I added in #56.

Victory!