Add Travis script to build+serve book from master

[CAD97]

See https://github.com/crate-ci/cargo-ghp-upload for more about the ghp-upload tool.

Build will end up at https://rust-hosted-langs.github.io/book/master/.

This requires Travis to have write-access to your repository. The simplest (and reasonably secure) way to achieve this is to create a Persional API Access Token with public_repoo scope. Then on Travis, define the secure environment variable GH_TOKEN with the value being the new token.

If you want to provide more scoped access, you can use a deploy key for repo-specific access. If no token is provided, the script will use SSH to clone from and write to the repository. Travis Pro handles the deploy key automatically, and free users can use Travis encrypt-file plus a script to move the private key into the correct location.

This also means that cargo ghp-upload works locally so long as you have ssh set up for your account. Branch and origin context are collected from Git instead of the CI environment. This means that cargo ghp-upload will work on CI other than Travis, but you currently have to manually prevent it from running on PR builds if you don't wan't it to.

[pliniker]

This is great, thanks!