[digestive-functors-heist] Bump heist upper bound

Support 0.14.x. I've not tested this in practice so I don't know if we need other changes, but it *builds* fine for me with this change.
Merge pull request #102 from CodeBlock/patch-1

[digestive-functors-heist] Bump heist upper bound
Version bump (digestive-functors-heist-0.8.6.1)
workaround for issue snapframework/snap-core#159