Version 3.18.3

• Aws\Api - Fixed parsing empty responses
• Aws\CognitoIdentityProvider - Remove non-JSON operations.
• Aws\Ec2 - Added support for ClassicLink over VPC peering
• Aws\Ecr - This update makes it easier to find repository URIs,
  which are now appended to the #describe_repositories, #create_repository,
  and #delete_repository responses.
• Aws\S3 - Added support for Post Object Signature V4
• Aws\S3 - Fixed Content-MD5 header for PutBucketReplication