Bug: `wp plugin install` installs a plugin when it shouldn't

I did: `wp plugin install`

I saw:

```
hmn-vagrant ➜  wordpress-trunk.dev  wp plugin install
PHP Notice:  Undefined offset: 0 in /home/vagrant/.wp-cli/vendor/wp-cli/wp-cli/php/WP_CLI/CommandWithUpgrade.php on line 101
Installing BFT Autoresponder (2.0)
Downloading install package from http://downloads.wordpress.org/plugin/bft-autoresponder.zip...
Unpacking the package...
Installing the plugin...
Plugin installed successfully.
```

I expected: to see usage instructions.