Rename ipfsHash to hashDigest in Contributor

naming should be consistent with the other contracts and digest is more
correct.

lib/contracts/contributor.js

@@ -20,11 +20,6 @@ class Contributor extends Base {
 
   getById(id) {
     return this.functions.getContributorById(id)
-      .then((data) => {
-        // TODO: remove when naming updated on the contract
-        data.hashDigest = data.ipfsHash;
-        return data;
-      })
       // Fetch IPFS data if available
       .then((data) => {
         return this.ipfs.catAndMerge(data, ContributorSerializer.deserialize);