Phoenix-letter

- Now Phoenix Letter is able to handle messages with no MessageAttributes. (kudos to WafflesMcDuff for the PR!)
- Updated libs
- Improved test coverage

- Now Phoenix Letter is able to handle messages with no MessageAttributes. (_kudos to WafflesMcDuff for the PR!_)

* Added the flag `--aws-keys` to securely ask for the AWS credentials to avoid keeping sensitive information accessible in plain text. Without this flag, it fallback to the [Boto3 default credential search](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html#configuring-credentials).

* fixed bugs during pip installation

* fixed bugs during installation through pip on python2.7

* bugfixes