Pass AWS credentials to automated tests

[irvinlim]

There should be a way for automated tests to access the AWS SDK with credentials.

Normally on AWS Lambda, you can specify the IAM role, but through docker-lambda, we can pass the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables using the addEnvVars option, which has to be a IAM user.

For passing this to the test runner, Travis accepts environment variables which will be forwarded to process.env. We can use dotenv, which will work for local test runs as well.