I don’t use the RFXtrx component myself, but looking to the docs, you might be able to work-around this with a dummy device (created during Travis install, like the secrets file).
If the check-config actually tries to connect to the device and requires a correct response, then I’m afraid I can’t help you with that…
Edit: I’ve also just tried to integrate the RFXtrx component in my configuration, and for some reason it does work for me, perhaps I’m missing something though
Edit 2: Adding the switch as RFXtrx does indeed show the error and creating something at the device’s location doesn’t seem to work. Maybe somebody else might know how to mock this device?
Edit 3: Final edit 
apparently HA doesn’t install the pyRFXtrx module. Adding pip install pyRFXtrx to your installation procedure of Travis seems to fix it.