Remove superflous quotes when dealing with inline terraform expressions

[flosell]

(triggered by #13)

When writing policy JSONs inline as heredoc, one often includes terraform expressions for certain values, e.g.

"Resource": ["${aws_kms_key.key.arn}"]

Currently, those are converted into normal string arrays with interpolations, e.g.

resources = ["${aws_kms_key.key.arn}"]

Since terraform 0.12, there's a more elegant way to express this, would be nice for iam-policy-json-to-terraform to render them:

resources = [ aws_kms_key.key.arn ]