Add support for bundled rake

Projects using Gemfiles are always tied to a specific version of rake.
By default, calling `rake` will use whatever the highest version of rake installed is.
If this version conflicts with the one in the Gemfile.lock, the rake task will abort with

```
rake aborted!
Gem::LoadError: You have already activated rake 10.4.1, but your Gemfile requires rake 10.3.2. Prepending `bundle exec` to your command may solve this.
```

If a project has a Gemfile, we can assume they will want to run `bundle exec rake`, not `rake`.