Update rbase64 encode usage (#24)
Update rbase64 encode usage (#24)

Based on the [documentation](https://github.com/boseji/rBASE64#example-1--using-the-standard-object) 
proper usage of rbase64 to encode is:
```
rbase64.encode("Hello world");
rbase64.result();
```
GoogleCloudPlatform/google-cloud-iot-arduino,GoogleCloudPlatform/google-cloud-iot-arduino,GoogleCloudPlatform/google-cloud-iot-arduino