Update Amazon EC2 Docker installation instructions

The installation guide for EC2 is outdated, as the current version of Amazon Linux (2014.09) is now Docker ready. No need to go through the manual route anymore. The official AMI has Docker packages in the repository now (this was the 'pre-release' option in the outdated instructions).

Docker-DCO-1.1-Signed-off-by: David Mat <david@davidmat.com> (github: davidmat)