Get default S3 credentials from DefaultAWSCredentialsProviderChain

As mentioned in the commit message, this fixes seratch/AWScala#11

Very simple - it just uses the DefaultAWSCredentialsProviderChain when calling awscala.s3.S3()

The original usage of specifying credentials with environment variables still works fine, just need a slightly different environment variable name for the secret to match Amazon conventions.
Sure but what else do you want me to change?  I've already changed the name of the environment variable on this line.