Make isProofRequired() work for the current block

The block hash of the current block is not known yet, so
we use the block hash of the previous block to determine
whether a proof is required for a block.

contracts/Proofs.sol

@@ -50,7 +50,7 @@ contract Proofs {
     internal view
     returns (bool)
   {
-    bytes32 hash = blockhash(blocknumber);
+    bytes32 hash = blockhash(blocknumber - 1);
     return hash != 0 && uint(hash) % periods[id] == markers[id];
   }
 

test/Proofs.test.js

@@ -45,7 +45,7 @@ describe("Proofs", function () {
     }
 
     async function minedBlockNumber() {
-      return await ethers.provider.getBlockNumber() - 1
+      return await ethers.provider.getBlockNumber()
     }
 
     async function mineUntilProofIsRequired(id) {