Use only the S3 and EC2 parts of the AWS Java SDK

Since 1.9.0 the AWS SDK is available not just as a big blob, but also split into per-service modules. As the elasticsearch-cloud-aws plugin only requires s3 and ec2 it shouldn't pull in all other parts.

This reduces the size of the release artifact from ~17M to ~5M.

Note that this depends on #137, but is based on master now (i.e. it won't compile :D)