feat: use ursa-optional and add it as dependency

lib/rsa-ursa.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var ursa = require('ursa');
+var ursa = require('ursa-optional');
 
 function notToJson() {
   return undefined;

package.json

@@ -39,7 +39,8 @@
   },
   "homepage": "https://git.coolaj86.com/coolaj86/rsa-compat.js#readme",
   "dependencies": {
-    "node-forge": "^0.6.41"
+    "node-forge": "^0.6.41",
+    "ursa-optional": "^0.9.4"
   },
   "trulyOptionalDependencies": {
     "buffer-v6-polyfill": "^1.0.3",