Skip S3 credentials file if AWS credentials are not provided

This prevents an invalid exhibitor.s3.properties file from being written and
used if the credentials are missing.

The bonus side-effect (that I'm actually interested in) is that the
s3credentials option is omitted, and Exhibitor can fall back onto IAM
profiles on EC2 instances.
Merge pull request #52 from groupme/guard-s3credentials

Skip S3 credentials file if AWS credentials are not provided