Support version flag when getting with `go get github.com/danielgtaylor/apisprout`

Thank you for creating this great tool.

I would like to see version of apisprout when I get it with `go get github.com/danielgtaylor/apisprout
`.

Currently, I got `Error: unknown flag: --version` since a binary from `go get` isn't generated from `govvv build` in [release.sh](https://github.com/danielgtaylor/apisprout/blob/e14cbf5e9ce2d352d206f3728ba12c0913fcd16b/release.sh)

Step to encounter the unkown flag error.

```
$ go get github.com/danielgtaylor/apisprout
$ which apisprout
/my-GOPATH/go/bin/apisprout
$ apisprout --version
Error: unknown flag: --version
Usage:
.
.
.
```