c4.large requires a VPC

Using the latest code from `master` and following the README.md, I get the following error due to `c4.large` requiring a VPC.

```ERROR: Stack named 'awseb-e-XXXXXXXXX-stack' aborted operation. Current state: 'CREATE_FAILED' Reason: The following resource(s) failed to create: [AWSEBAutoScalingGroup].
ERROR: Creating Auto Scaling group named: awseb-e-XXXXXXXXX-stack-AWSEBAutoScalingGroup-LPMU6SAD3K5W failed. Reason: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request. Launching EC2 instance failed.
```
Changing the command to a `c3.large` succeeds, e.g.:

`eb create -i c3.large --scale 1 --envvars TARGET_URL= --instance_profile aws-elasticbeanstalk-locust-role`