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/)
2017/05/22 10:38:19 Generated command/internal_plugin_list.go
==> Removing old directory...
==> Building...
Number of parallel builds: 3
--> linux/amd64: github.com/hashicorp/terraform
==> Results:
total 209M
-rwxr-xr-x 1 jake jake 209M May 22 10:39 terraform
$ terraform version
Terraform v0.9.6-dev (fd472e4a86500606b03c314f70d11f2bc4bc84e5+CHANGES)
```
`TF_RELEASE` is set (mimicking the `make bin` target):
```sh
$ TF_RELEASE=1 make dev
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/22 10:40:39 Generated command/internal_plugin_list.go
==> Removing old directory...
==> Building...
Number of parallel builds: 3
--> linux/amd64: github.com/hashicorp/terraform
==> Results:
total 121M
-rwxr-xr-x 1 jake jake 121M May 22 10:42 terraform
$ terraform version
Terraform v0.9.6
```
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