Manually grab `ace` to use latest gitbook

This makes gitbook 1.5.0 (what NPM installed) work for me. I had an syntax highlighting issues with 1.4.2 which is what is installed by Ubuntu but I think it's related to `gitbook serve` not working properly. Otherwise, it seems to work fine.

I wasn't sure if you'd want *ace* in tree so I downloaded it in the script.