pin the version of nano to avoid breaking changes (#97)

diff --git a/package.json b/package.json
index 90a13a7..becec8e 100755
--- a/package.json
+++ b/package.json
@@ -9,7 +9,7 @@
     "cron": "^1.1.0",
     "express": "^4.13.4",
     "lodash": "^4.5.0",
-    "nano": "^6.2.0",
+    "nano": "6.3.0",
     "request": "^2.69.0",
     "winston": "^2.1.1",
     "moment": "^2.12.0",