Update nrf51-sdk dependency to v2.1.0

This is required because 2.1.0 includes additional files from the peer manager
that are used by the implementation of the whitelisting API to check whether an
address can be generated from a given IRK.

module.json

@@ -25,7 +25,7 @@
   ],
   "dependencies": {
     "ble": "^2.3.0",
-    "nrf51-sdk": "^2.0.0"
+    "nrf51-sdk": "^2.1.0"
   },
   "extraIncludes": [
     "source/btle",