Remove SLACK_TOKEN from environment of containers

[rstata]

The Slack token used by Limbo is passed via the SLACK_TOKEN environment variable. When running in ECS, this variable can be inspected with the ECS Web UI. A better approach would be to store the token in S3 or some other storage service that provides access control, and then use the IAM role provided by issue #22 to gain access to the token. (This would require a change to the initialization code of Limbo to provide an alternative mechanism for loading the Slack token. This alternative mechanism should be available through some sort of "feature flag," with the current environment-based initialization configured as the default.)