Add support for node 14.x

[Bamieh]

AWS now supports Node.js 14. This PR allows this package to support this node version https://aws.amazon.com/blogs/compute/node-js-14-x-runtime-now-available-in-aws-lambda/

• [x] Tested lambda-tester locally using node.js version 14.18.2
• [x] Tested the lambda-tester is compatible with nodejs 14.18.2 by using LAMBDA_TESTER_NODE_VERSION_CHECK=false on a real-world usecase
• [x] Travis will run tests using node.js 14.x to make sure we are compatible.
• [x] Added a .node-version file to encourage development on this package using the latest compatible node.js version

Closes https://github.com/vandium-io/lambda-tester/issues/65