Event files are not in the same order

See https://github.com/aws/aws-lambda-java-libs/pull/216

Event files are not taken in the same order. `Files.list`(in [HandlerParamsArgumentsProvider](https://github.com/aws/aws-lambda-java-libs/blob/master/aws-lambda-java-tests/src/main/java/com/amazonaws/services/lambda/runtime/tests/HandlerParamsArgumentsProvider.java))  does not guarantee ordering. Must be explicitly sorted.