auto-scaling: Correct caveats

The currently formula caveats uses the [AmazonWebServicesFormula caveats](https://github.com/Homebrew/homebrew/blob/c5ea5dfe23d5c96a34c331261fbab50feb641d13/Library/Homebrew/formula_specialties.rb#L30-L44) which recommend adding AWS_ACCESS_KEY and AWS_SECRET_KEY.

These will not work. You must provide a credential file as per the instructions in the auto-scaling cli instllation guide:

http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/UsingTheCommandLineTools.html

> After you sign up for AWS, you must create access keys for the account. Your access keys consists of an Access Key ID and a Secret Access Key. You must provide your access keys to the command line tool so that they know that the commands that you issue come from your account. <strong>The command line tool reads your access keys from a credential file that you create on your local system.</strong>
>
> You can either specify your credential file with the --aws-credential-file parameter every time you issue a command, or you can create an environment variable that points to the credential file on your local system. If the environment variable is properly configured, you can omit the --aws-credential-file parameter when you issue a command. The following procedure describes how to create a credential file and a corresponding AWS_CREDENTIAL_FILE environment variable.

Note that the EC2 command line tools still appear to use the other environment variables:

http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/set-up-ec2-cli-linux.html

> The easiest way to provide your access keys to the Amazon EC2 CLI is to set the AWS_ACCESS_KEY and AWS_SECRET_KEY environment variables.