[beta] Backport book changes for rustup#38415

Merged

alexcrichton merged 3 commits intorust-lang:betafrom

brson:beta-next

Dec 16, 2016

Commits on Dec 16, 2016

Book: rustup.sh -> rustup.rs

steveklabnik
authored and
brson
committed
Update book for rustup
brson
committed
Bump beta prerelease version
brson
committed