support executing inside AWS CodeBuild

[theburningmonk]

When used inside a AWS CodeBuild build environment (which uses ECS under the hood) one needs to use the AWS_CONTAINER_CREDENTIALS_RELATIVE_URI environment variable and fetch the credentials from http://169.254.170.2${process.env.AWS_CONTAINER_CREDENTIALS_RELATIVE_URI}, which returns a payload that looks like this:

{  
  "RoleArn": "...",
  "AccessKeyId": "...",
  "SecretAccessKey": "...",
  "Token": "...",
  "Expiration": "2017-12-24T03:14:03Z"
}

see here for a related question which pointed me to the above URL.

[mhart]

Fixed by #3