Update Arduino-mbed core to v4.0.6

Author: valeros

platform.json

@@ -65,7 +65,7 @@
       "type": "framework",
       "optional": true,
       "owner": "platformio",
-      "version": "~4.0.2"
+      "version": "~4.0.6"
     },
     "framework-cmsis": {
       "type": "framework",