New Heroku-16 stack available

[nateberkopec]

Not sure if we actually need to change anything for this or not

[sethfitz]

May need to recompile, in which case multiple versions will need to exist, e.g. https://github.com/mojodna/heroku-buildpack-cairo/blob/master/bin/compile#L45

[nateberkopec]

Heroku says it "should work" with buildpacks for Cedar, but who knows.

[brian-kephart]

I've been using this buildpack on Heroku-16 for a couple months. It works just fine.