Fbs

Otherwise, their results may depend on the user's operating system.

Thank you weiyi0 for the PR (250).

The problem was that version numbers need to consist of three numbers (eg. `1.2.3`). Now, there is a meaningful error message when the user tries to enter just `1.0`.

fbs's `buildvm fedora` and `buildvm arch` commands were broken. This releases fixes Fedora and gives a base implementation towards a fix for Arch. Similarly to the previous release (which was for Ubuntu), you can now specify the Python version (and Python OS-level build dependencies) via additional `build_args` settings called `python_version` and `python_build_deps`.

Motivated by PR 231. Thanks meramsey!

You can now also specify the Python version you want in the VM by specifying the `build_arg` called `python_version`. See `ubuntu/Dockerfile` and `base.json` in `_defaults/`.

Thank you lune for the [PR](https://github.com/mherrmann/fbs/pull/238)!

Thank you pm990320 for the PR (234)!