Fixed promise type in TS typings

Author: vincekovacs

package-lock.json

@@ -30,6 +30,7 @@
     "jwk"
   ],
   "dependencies": {
+    "@types/bluebird": "^3.5.8",
     "bluebird": "3.5.0",
     "debug": "2.6.3",
     "jsrsasign": "7.1.2",

package.json

@@ -1,3 +1,5 @@
+import * as Promise from "bluebird"
+
 declare namespace SimpleKeystore {
     class KeystoreError extends Error {
         constructor(message: string);