Feature Request: Provide method for getting license list version

Currently the SPDX format has a field for the license list version used. It would be nice if there was a constant/method to get that. It looks like the license is built at a commit? not sure if that can be used in the SPDX format, so i guess with this it would be helpful if only released versions were used.
Something like:
```
Spdx::LIST_VERSION
```
would be helpful
