Standardise which environment variables we use

The following tools all use AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY,
so let's use them too:

* aws-cli      : https://github.com/aws/aws-cli#getting-started
* aws-sdk-ruby : http://docs.aws.amazon.com/AWSRubySDK/latest/index.html#Basic_Configuration
* aws-sdk-js   : http://docs.aws.amazon.com/AWSJavaScriptSDK/guide/node-configuring.html#Credentials_from_Environment_Variables