Fix "method_complexity" issue in lambda_handlers/handlers/lambda_handler.py

[sleistner]

Function __call__ has a Cognitive Complexity of 6 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/enter-at/python-aws-lambda-handlers/lambda_handlers/handlers/lambda_handler.py#issue_5fa57dd097fc8c0001000043