Show help to invoke uninstall without VERSION

[giginet]

Currently, we invoke uninstall command without any arguments. then following error is shown.

$ xcversion uninstall
[!] Version  is not installed.

This PR make it to show the help message in this situation.

$ xcversion uninstall
[!] A VERSION argument is required.

Usage:

    $ xcversion uninstall VERSION

      Uninstall a specific version of Xcode.

Options:

    --verbose   Show more debugging information
    --no-ansi   Show output without ANSI codes
    --help      Show help banner of specified command

[coveralls]

Coverage increased (+1.3%) to 61.35% when pulling 108922d on giginet:uninstall-help into 4c7d27b on neonichu:master.

[coveralls]

Coverage increased (+0.9%) to 60.941% when pulling b15352e on giginet:uninstall-help into 4c7d27b on neonichu:master.

[neonichu]

Thanks for the contribution!