update AWS environment variables

The `aws` program looks for the following environment variables to be set:

* `AWS_ACCESS_KEY_ID`
* `AWS_SECRET_ACCESS_KEY `

Source: http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html

I've confirmed this to work on a new EC2 instance, using the Amazon AMI (AWS CLI 1.4.2).