Update graft algorithm with graft fork support

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/