Fix Travis-CI `getDeviceList` error
Fix Travis-CI `getDeviceList` error

`Fatal error: Cannot read property 'getDeviceList' of undefined` causes
because no switch usb is there.

tessel/t2-cli,tikurahul/t2-cli,nodebotanist/t2-cli,tessel/t2-cli,LinusU/t2-cli,rwaldron/t2-cli,nodebotanist/t2-cli,rwaldron/t2-cli,nodebotanist/t2-cli,Student007/t2-cli,LinusU/t2-cli,rwaldron/t2-cli,tikurahul/t2-cli,tikurahul/t2-cli,tessel/t2-cli