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...