fix(utils): make util.toBase64 browserify compatible

`bn.toArrayLike` is used instead of `bn.toBuffer`, to ensure compatibility with browserify.

test/util.spec.js

@@ -0,0 +1,21 @@
+/* eslint max-nested-callbacks: ["error", 8] */
+/* eslint-env mocha */
+'use strict'
+
+const expect = require('chai').expect
+const util = require('../src/crypto/util')
+const BN = require('bn.js')
+
+describe('Util', () => {
+  let bn
+
+  before((done) => {
+    bn = new BN('dead', 16)
+    done()
+  })
+
+  it('toBase64', (done) => {
+    expect(util.toBase64(bn)).to.be.eql('3q0')
+    done()
+  })
+})