resolves #1671 install Kindlegen so .mobi output can be produced

[slonopotamus]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

[HonkingGoose]

Oh there's a spelling error in the commit message:

Benefits:
1. Command is printed to stdout, so it is clear what is bein execured
2. Whole build will fail if any command fails

Might want to force-push a better commit message?

Benefits:
1. Command is printed to stdout, so it is clear what is being executed
2. Whole build will fail if any command fails

[slonopotamus]

Whoops. Fixed.

[HonkingGoose]

I'd say this is ready for review by @ben now. 😉

[ben]

✨

[HonkingGoose]

That seems to have done the trick, we're publishing .mobi files again! 🚀

Thank you very, very, very much @slonopotamus for your hard work on getting the kindlegen gem working again, and for your help with getting our .mobi build working again! 🎉 ✨ ✨