Update the test configuration

- Add additional node versions for Travis to test.
- Change var to const in mocha JS file.

.travis.yml

@@ -1,5 +1,8 @@
 language: node_js
 node_js:
-  - 4
+  - "node"
-  - 6
+  - "6"
-  - 8
+  - "7"
+  - "8"
+  - "9"
+  - "10"

test/base58.test.js

@@ -1,6 +1,6 @@
-var assert = require("assert");
+const assert = require("assert");
-var examples = require("./examples");
+const examples = require("./examples");
-var base58 = require("..");
+const base58 = require("..");
 
 function exampleRunner(callback) {
   Object.keys(examples).forEach(function(str) {