microbit: Added reference to nrf51-sdk

ARM mbed have extracted the Nordic SDK from their BLE libraries, but dependencies still exist...
Maintaining a local branch here to provide the opportunity for tailoring to reduce RAM overhead.

module.json

@@ -10,9 +10,10 @@
   ],
   "dependencies": {
       "mbed-classic": "~0.0.4",
-      "ble": "lancaster-university/BLE_API#master",
-      "ble-nrf51822": "lancaster-university/nrf51822#master",
-      "microbit-dal":"lancaster-university/microbit-dal#master"
+      "ble": "lancaster-university/BLE_API",
+      "ble-nrf51822": "lancaster-university/nrf51822",
+      "microbit-dal":"lancaster-university/microbit-dal",
+	  "nrf51-sdk":"lancaster-university/nrf51-sdk"
   },
   "targetDependencies": {},
   "bin": "./source"