Use newer rustbook version to improve formating for the hosted pages at https://stevedonovan.github.io

[tim-seoss]

Hi,

Thanks for hosting and writing the gentle intro!

I noticed that https://stevedonovan.github.io/rust-gentle-intro is built with an older version of rustbook. Newer versions improve the clarity of various things like controls and code samples, and you may wish to consider rebuilding to take advantage of these.

Thanks!

Tim.