Updates master node lookup to use new dvid api

The old master lookup used an old branches/key/master keyvalue that had
to be added manually. This update switches to the
/api/repo/{id}/branch-versions/master endpoint, which is the correct way
moving forward.

src/lib/dvid.js

@@ -46,7 +46,7 @@ export function createRepo(options) {
 }
 
 export function masterUUID(rootUUID) {
-  return api.node({uuid: rootUUID, endpoint: 'branches/key/master'});
+  return api.repo({uuid: rootUUID, endpoint: 'branch-versions/master'});
 }
 
 export function getDefaultInstances(uuid) {