microbit: updated module.json to point to changes in our mbed-classic repo

module.json has now been updated to include changes to the interrupt
priority of the system ticker, as there were some interoperability
issues with the nordic softdevice stack.

module.json

@@ -14,7 +14,7 @@
   "author": "Joe Finney <j.finney@lancaster.ac.uk (mailto:j.finney@lancaster.ac.uk) >",
   "homepage": "https://developer.mbed.org/teams/Lancaster-University/code/microbit/",
   "dependencies": {
-    "mbed-classic": "lancaster-university/mbed-classic#microbit_hfclk",
+    "mbed-classic": "lancaster-university/mbed-classic#microbit_hfclk+mb1",
     "ble": "lancaster-university/BLE_API#v2.5.0+mb3",
     "ble-nrf51822": "lancaster-university/nrf51822#v2.5.0+mb5",
     "nrf51-sdk": "lancaster-university/nrf51-sdk#v2.2.0+mb3"