Add expect for previousHash to addBlock test

[delventhalz]

Fixes #96

Ensures that students set the previousHash properly when adding blocks to the blockchain. Previously this was not checked.

To test:

cd code/part-one/
npm install
npm test

These tests should all pass. Then if you modify line 140 of code/part-one/blockchain.js so it reads:

    const block = new Block(transactions, this.getHeadBlock().previousHash);

This is an error, and if you run npm test again, it should be caught by the tests. Previously it would have been missed.

[therobinkim]

lgtm -- tested & confirmed working on my end!