This commit changes the Travis configuration to use the shared infra
configuration to upload the build to GitHub pages. This has a few
advantages:
- The repository will benefit from improvements made to the shared
configuration without needing any change.
- The history of the gh-pages branch will be overridden each build,
reducing the clutter.
Spoke with someone at Rust Belt Rust who mentioned this idea (@mre 👋)
and decided to take a stab at it. I originally implemented this upstream
in the Rust source repository (by adding a git submodule and dynamically
generating the contents of the book there). But then then I spoke with
@steveklabnik and agreed that doing it in CI in this repository would be
a lot easier.