Corrected secret ley variable name

According to http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html, the env variable must be named AWS_SECRET_ACCESS_KEY, not AWS_SECRET_KEY.
Merge remote-tracking branch 'upstream/master' into patch-7

Conflicts:
	README.md
Merge pull request #8 from ulsa/patch-7

Corrected secret key variable name