buildbox-casd --version doesn't work
I looked into this today in order to solve BuildStream's need to [verify the required version of BuildBox at startup](https://github.com/apache/buildstream/issues/1189).
I have a few issues here:
* I am unable to find where the *latest builds of buildbox* are located
* BuildStream's [documentation](https://docs.buildstream.build/master/main_install.html#install-buildbox) currently only links specifically to [BuildBox 0.0.38](https://buildbox-casd-binaries.nyc3.cdn.digitaloceanspaces.com/buildbox-x86_64-linux-0.0.38-c91dd9b9.tar.xz) which I presume is probably quite old.
* BuildBox 0.0.38 supports the `--version` option according to `--help`, but bails out with this error if given the `--version` option:
```
[buildboxcommon_commandline.cpp:414]: parse error: required positional argument(s) missing from command line
```
* The version I happen to have installed on my machine is more recent, I think @juergbi gave me the link to the new version after fixing an issue early this year, I'm not sure what version it is, but the result of typing `./buildbox-casd --version` yields the following results:
```
tristan@pony:~$ buildbox-casd --version
buildbox-casd unknown
```
If this is already fixed in more recent versions, please go ahead and close this (and let me know where I can go to find the latest binaries).
issue