Deeply Nested Placeholder keys are not replaced

[xpaulnim]

For instance, in the following

{
  "InputDataConfig": [
    {
      "ChannelName": "training",
      "DataSource": {
        "S3DataSource": {
          "S3DataDistributionType": "FullyReplicated",
          "S3DataType": "S3Prefix",
          "S3Uri": "$.Payload.cloned_config"
        }
      }
    }
  ]
}

where S3Uri is either ExecutionInput or StepInput should be replaced with S3Uri.$

[wong-a]

Should be addressed by #142

[shivlaks]

#142 has been merged and will be included in the next released version (v2.2.0)