Travis test for slip-radio

An attempt to test slip-radio with Travis in 13-slip-radio.

01-sky-slip-radio-dio tests 2 things:
* That commands can be interpreted from stdin
* That a packet is successfully sent using !S and properly decoded at the receiver (I use a DIO and check when the DAG is found).

The COOJA simulation was tested locally, but I'm not certain about the whole integration in Travis. The build will tell us :)