pin ArduinoJson version to 6.20.1

Apparently this issue bblanchon/ArduinoJson/issues/1914 is biting us.
Fixes issue #24.

Signed-off-by: Benjamin Cabé <kartben@users.noreply.github.com>

firmware/platformio.ini

@@ -47,5 +47,5 @@ lib_deps =
     arduino-libraries/NTPClient
     knolleary/PubSubClient
     hideakitai/MsgPack
-    bblanchon/ArduinoJson
+    bblanchon/ArduinoJson@6.20.1
 	; END   Azure IoT dependencies