Log Message:
asdf: update to 0.16.3. Changes:

* add missing version command
* correct formatting of version in "already installed" error message
* correct typo in `Upgrading to 0.16.0` documentation
* don't error if version already installed
* pass environment variables through to `exec-env` callback
* repair invalid fish shell completion code
* return no error from shims.RemoveAll when shims dir missing