Removed checks for ENV vars and let the AWS SDK figure how to authenticate

The AWS SDK is pretty smart about using a few different ways to configure itself.  By default, it will use the same ENV vars that you look for, but will also use IAM roles if they are enabled. That means that this PR should fix https://github.com/clarete/s3sync/issues/15 also.

For more information about how the AWS SDK looks for credentials, please see this:
http://docs.aws.amazon.com/AWSSdkDocsRuby/latest/DeveloperGuide/ruby-dg-roles.html