randombytes: Replace run-time require detection

Using require() as an expression leads to compile errors in webpack.
This issue can be solved by guarding the use of require by the 'correct'
run-time check using `typeof require`.

randombytes.js

@@ -20,7 +20,7 @@ var randombytes = (function () {
 
   if (crypto && crypto.getRandomValues) return browserBytes
 
-  if (require != null) {
+  if (typeof require === 'function') {
     // Node.js. Bust Browserify
     crypto = require('cry' + 'pto')
     if (crypto && crypto.randomBytes) return nodeBytes