Support Node v14

[VySellathurai]

Hi! First of all, thanks for your work.

AWS Lambda released support for Node 14.x on feb 2021!
https://aws.amazon.com/fr/blogs/compute/node-js-14-x-runtime-now-available-in-aws-lambda/

Could you make the change pls ? (based on the pr : #55)

I'm using your option LAMBDA_TESTER_NODE_VERSION_CHECK=false until it will support the new version.