TTML2 Default Branch, New ED Link

I have changed the default branch from master to gh-pages, to allow
mirroring into the github.io space. The former master branch has been
deleted.

The new "Latest Editor's Copy" URI is now [1], which serves up the copy in
the gh-pages branch.

[1] https://w3c.github.io/ttml2/spec/ttml2.html

For non-gh-pages branches, e.g., those used for PRs, etc., you will need to
continue to use the rawgit.com entry point, e.g., [2], substituting
*branch-name* with the desired value.

[2] http://rawgit.com/w3c/ttml2/*branch-name*/spec/ttml2.html