The revision field is only populated on dev builds so this means
most releases of Terraform have an empty "terraform_revision" field
in the JSON output. Since we recommend developers use go tooling
to `go build` this tool when developing, the revision is not useful
data and so it is removed.

Now that we're verifying the Terraform version during state loading, we
need to force a particular Terraform version to use during these tests.

Update all references to the version values to use the new package.
The VersionString function was left in the terraform package
specifically for the aws provider, which is vendored. We can remove that
last call once the provider is updated.

Instead of using a hardcoded version prerelease string, which makes release automation difficult, set the version prerelease string from an environment variable via the go linker tool during compile time.

The environment variable `TF_RELEASE` should only be set via the `make bin` target, and thus leaves the version prerelease string unset. Otherwise, when running a local compile of terraform via the `make dev` makefile target, the version prerelease string is set to `"dev"`, as usual.

This also requires some changes to both the circonus and postgresql providers, as they directly used the `VersionPrerelease` constant. We now simply call the `VersionString()` function, which returns the proper interpolated version string with the prerelease string populated correctly.

`TF_RELEASE` is unset:

```sh
$ make dev
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/2...