Fix unit test failure at travis (#198)
Fix unit test failure at travis (#198)

I got the following error
'Node.js version v0.10.48 does not meet requirement for yarn. Please use
Node.js 4 or later.'
motdotla/node-lambda,motdotla/node-lambda,DeviaVir/node-lambda,teebu/node-lambda,DeviaVir/node-lambda,teebu/node-lambda