Show usage in plugins:install/uninstall commands when an argument is not provided

Right now a TypeError (no implicit conversion of nil into String) is raised

```
 % heroku plugins:install
 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       no implicit conversion of nil into String (TypeError)
    Backtrace:   /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/plugin.rb:159:in `basename'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/plugin.rb:159:in `guess_name'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/plugin.rb:98:in `initialize'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/command/plugins.rb:41:in `new'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/command/plugins.rb:41:in `install'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/command.rb:218:in `run'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/lib/heroku/cli.rb:28:in `start'
                 /Users/guille/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/heroku-3.2.2/bin/heroku:17:in `<top (required)>'
                 /Users/guille/.rbenv/versions/2.1.0/bin/heroku:23:in `load'
                 /Users/guille/.rbenv/versions/2.1.0/bin/heroku:23:in `<main>'

    Command:     heroku plugins:install
    Plugins:     heroku-config-versions
    Version:     heroku-gem/3.2.2 (x86_64-darwin13.0) ruby/2.1.0
```

Usually I run the commands without arguments when I want to know the correct usage.