Use bundle exec to invoke rake

`rake` should be invoked with `bundle exec`, because the system rake version could be different from the bundled one or the rake tasks could depend on different gem versions than the ones globally installed.

But I could not think of a nice fallback in case that bundler is not used. If someone has an idea, I will encorporate it. I could check on the presence of the settings from #capistrano/rails and create a global helper function `bundle exec` but that does not sound very good. Maybe there is some idiomatic rake way.