Restrict to node version 10

Many unit tests fail under node 12. It is possible that some of these
are caused by issues in node that will be fixed in the future.

This also removes the dependency on 'assert-diff' since it is not
necessary. It can easily be re-added in the future, if needed.

packages/ripple-binary-codec/test/shamap-test.js

@@ -1,4 +1,4 @@
-const assert = require('assert-diff');
+const assert = require('assert');
 const {ShaMap} = require('../src/shamap.js');
 const {binary: {serializeObject}, Hash256, HashPrefix}
   = require('../src/coretypes');