fix(package): update ioredis to version 4.9.1

Closes #12

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
   },
   "dependencies": {
     "bluebird": "^3.5.0",
-    "ioredis": "^3.1.4"
+    "ioredis": "^4.9.1"
   },
   "engines": {
     "node": ">=6.10.0"