Bump go-tfe to 0.3.13

[justincampbell]

Fixes #71

[sudomateo]

Do we need to change the value attribute from required to optional?
https://github.com/terraform-providers/terraform-provider-tfe/blob/master/tfe/resource_tfe_variable.go#L29-L33

[justincampbell]

@sudomateo I think if it's Required then an empty string "" would still be allowed. Is there a reason to allow it to be omitted other than for API consistency?

[sudomateo]

@justincampbell That makes sense to me. Leaving it required would mean the value would either be "" or some non-empty string. I don't have any other reason besides API consistency.

[svanharmelen]

It's a shame you already tagged a new release for the go-tfe package, as now we released some code that was waiting for a PR (hashicorp/go-tfe#62) to update/fix the newly added Notification API (which wasn't released yet).

I'm still going to merge that PR, but that means version 0.3.12 and 0.3.13 will now be backwards incompatible for a small bit. I'll tag a new release in a minute, so please use that version instead in this PR.

[svanharmelen]

I think if it's Required then an empty string "" would still be allowed. Is there a reason to allow it to be omitted other than for API consistency?

I prefer API consistency, so please make the value field optional and default to "" to express it has the exact same behavior as the API (and the docs).

[justincampbell]

@svanharmelen Updated to 0.3.13 and marked value as optional

[svanharmelen]

LGTM!