According to the doc about `config.toml` of containerd:
```
If no version number is specified inside the config file then it is assumed to
be a version 1 config and parsed as such.
```
However, it's not true recently.
This will break the backward-compatibility in some environment.
This commit fixes this issue.
Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>