No need to supply AWS_* configuration variables when running on EC2

[jcmuller]

Using the aws-sdk gem, you don't need to set the AWS_* environment variables since the IAM roles can be set at the instance level for your S3 buckets. Is there a way to replicate this with AssetSync?

[gmcnaughton]

@jcmuller #294 suggests that there is an aws_iam_roles config option that will tell asset_sync to look up credentials from the instance's role. I haven't tried it yet, but might be worth a look!

[oskarpearson]

This can be resolved.

aws_iam_roles does work as expected in my testing.

Outstanding PR https://github.com/rumblelabs/asset_sync/pull/307/files would add this to the documentation.

From our configs:

AssetSync.configure do |config|
    ...
    config.aws_iam_roles = true
end