Graft is forking soon, with updated software released yesterday, to what
will be v8 in Graft (current Graft cryptonight algorithm is graft v7).
This commit updates the `graft` algorithm to recognize the fork and
switch algorithms.

Fork annoucement:
https://www.graft.network/2018/04/12/patch-1-1-1-released-major-network-update-block-65110/