Replace AWS_SECURITY_TOKEN with AWS_SESSION_TOKEN

[radeksimko]

Amazon recently decided to stick to this name and use it as a convention in all AWS-related tools.

I think that supporting both variable names, the same way that botocore does is the way to go:
https://github.com/boto/botocore/blob/c76553d3158b083d818f88c898d8f6d7918478fd/botocore/credentials.py#L260-262

If you agree, I can send a PR.

[joestump]

Just got bit by this using terraform with AWS_SECURITY_TOKEN.

[radeksimko]

Terraform does not use this library anymore, we migrated over to the official AWS Go SDK instead during early 2015 (more than 2 years ago).