Fixes #8141 - correct instructions for testing in rake file

We can remove this. Does `rake test` work for you?

I only use `bundle exec` and I get:

```
[lzap@lzapx foreman_discovery]$ be rake test
/home/lzap/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/bundler-1.5.3/lib/bundler/rubygems_integration.rb:240:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
        from /home/lzap/.rbenv/versions/2.0.0-p247/bin/rake:22:in `<main>'
```

Adding it to Gemfile won't hurt, I suppose?