refactor: remove 'hello' messages and fix test suite (#54)

Author: coleHafner

README.md

@@ -30,7 +30,6 @@ Devices that send messages to nRFCloud can optionally have data displayed on car
 
 | MessageType      | Description | deviceToCloud or cloudToDevice |
 | :-----------:      | :----------- | :-----------:                    |
-| HELLO | Tells applications that device is ready to send data	  | C2D |
 | START | Tells application to start sending data to cloud	      | C2D |
 | STOP  | Tells device to stop sending data	                      | C2D |
 | INIT  | Tells the application to initialize or reset	          | C2D |

schemas/deviceToCloud/flip/flip-hello-example.json

@@ -10,7 +10,6 @@
        "messageType":{
           "type":"string",
           "enum":[
-             "HELLO",
              "DATA"
           ]
        },

schemas/deviceToCloud/flip/flip.json

@@ -10,8 +10,7 @@
        "messageType":{
           "type":"string",
           "enum":[
-              "DATA",
-              "HELLO"
+              "DATA"
           ]
        },
        "data":{