use ursa-optional or ursa

lib/generate-privkey-ursa.js

@@ -1,7 +1,13 @@
 'use strict';
 
 module.exports = function (bitlen, exp) {
-  return require('ursa').generatePrivateKey(bitlen || 2048, exp || 65537).toPrivatePem().toString('ascii').trim();
+  var ursa;
+  try {
+    ursa = require('ursa');
+  } catch(e) {
+    ursa = require('ursa-optional');
+  }
+  return ursa.generatePrivateKey(bitlen || 2048, exp || 65537).toPrivatePem().toString('ascii').trim();
 };
 
 if (require.main === module) {