Fix specification of elasticbeanstalk tier

According to the API: http://docs.aws.amazon.com/elasticbeanstalk/latest/api/API_CreateEnvironment.html
The ``member`` part of the Tier key names is not valid.
I was always getting:
``` js
{"Error":
    {"Code":"InvalidParameterValue",
     "Message":"Environment tier definition not found",
     "Type":"Sender"},
"RequestId":"xxxxx"}
```

This allows an environment to be created and to specify the tier_name and tier_type.
``` py
tier_name = 'Worker'
tier_type = 'SQS/HTTP'
environment = beanstalk.create_environment(application_name, environment_name, version_label=label,
                                            template_name=template_name, description=None,
                                            option_settings=None, options_to_remove=None,
                                            tier_name=tier_name, tier_type=tier_type, tier_version='1.0')
```