v0.15.1

This is based off of the OctoPi 0.15.1, and includes the "latest" of cnc.js (1.9.15) and octoprint (1.3.9).

I'm going to change my versioning to follow that of octopi, because it will help me keep things straight in my mind.

This is once again based on the raspian image from 2018-03-13. The latest one, from June, has a different wpa-supplicant file, and needs some work to get going. Running an update would be a good idea.