Switch to kroimon's Arduino-SerialCommand

The SerialCommand library included with Clyde has issues
parsing long strings and is generally flaky.  kroimon re-wrote
SerialCommand to be smaller and less buggy.

You can see the existing SerialCommand fail by doing sending
this command to Clyde:
SET_AMBIENT 100 150 200

In contrast, the kroimon one just works, and the code is much
cleaner and compact.  :+1:

See: https://github.com/kroimon/Arduino-SerialCommand
Merge pull request #6 from polastre/hotfix/SerialCommand

Switch to kroimon's Arduino-SerialCommand