Minimum AWS scraping role missing Elasticache read permissions

[corey-cole]

The minimal IAM role given in the README.md is missing permissions that are used in cache.py

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": ["ec2:DescribeInstanceTypes", "ec2:DescribeRegions", "pricing:*", "elasticache:DescribeEngineDefaultParameters"],
      "Resource": "*"
    }
  ]
}