Update dependency to mbed-core instead of mbed

Author: Seppo Takalo

module.json

@@ -19,7 +19,7 @@
     "source/include"
   ],
   "dependencies": {
-    "mbed": "~3.0.1",
+    "mbed-core": "~0.1.1",
     "mbed-client-libservice": "~2.0.9",
     "mbed-client": "~0.1.3",
     "mbed-6lowpan-eventloop": "~0.0.8",