Choose Polkadot commit and update to it

[ansermino]

We need to select a commit of Polkadot to target moving forward. We should establish what updates to the runtime we need to make (and possibly elsewhere).

We will need a genesis file to match. Doesn't necessarily need to match Kusama if we are testing on a local net.

[noot]

when we update the runtime, will need to change the AuraApi_authorites call to Grandpa_authorities. additionally, the Grandpa_authorities call takes a block number; need to update args

also need to update babe randomness to [sr25519.VrfOutputLength]byte from byte