appropriately handle registry source with no versions section

[johnnychen94]

Old jill versions don't require versions section for registry:

# ~/.config/jill/sources.json
{
    "upstream": {
        "LFLab": {
            "name": "LFLab@Math",
            "urls": [
                "https://mirrors.lflab.cn/julia/releases/$vminor_version/$filename"
            ],
            "latest_urls": [
                "https://mirrors.lflab.cn/julia/releases/latest/$latest_filename"
            ]
        }
    }
}

and it outputs:

jill install 1 --upstream LFLab --confirm
JILL - Julia Installer 4 Linux (MacOS, Windows and FreeBSD) -- Light
querying release information from None
Traceback (most recent call last):
  File "/usr/local/bin/jill", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/dist-packages/jill/__main__.py", line 19, in main
    }, name="jill")
  File "/usr/local/lib/python3.6/dist-packages/fire/core.py", line 141, in Fire
    component_trace = _Fire(component, args, parsed_flag_args, context, name)
  File "/usr/local/lib/python3.6/dist-packages/fire/core.py", line 471, in _Fire
    target=component.__name__)
  File "/usr/local/lib/python3.6/dist-packages/fire/core.py", line 681, in _CallAndUpdateTrace
    component = fn(*varargs, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/jill/install.py", line 387, in install_julia
    raise(ValueError(msg))
ValueError: wrong version(>= 0.6.0) argument: 1
Example: `jill install 1`

This bug should only exist in v0.9.0 and v0.9.1