fix(stubs): allow decode method to be stubbed (auth0#876)

* refactor: revert 15a1bc4 to to solve auth0#875, auth0#870, and possibly also auth0#876

Author: jakelacey2012

index.js

@@ -1,12 +1,8 @@
 module.exports = {
+  decode: require('./decode'),
   verify: require('./verify'),
   sign: require('./sign'),
   JsonWebTokenError: require('./lib/JsonWebTokenError'),
   NotBeforeError: require('./lib/NotBeforeError'),
   TokenExpiredError: require('./lib/TokenExpiredError'),
 };
-
-Object.defineProperty(module.exports, 'decode', {
-  enumerable: false,
-  value: require('./decode'),
-});