Using `bundle exec rake` instead of `rake`

Otherwise conflict of Rake versions is possible:

```
 INFO [7c96ac77] Running rake assets:precompile on 37.139.8.xxx
DEBUG [7c96ac77] Command: cd /home/deploy/navigator/releases/20130805155434 && ( RAILS_ENV=production rake assets:precompile )
DEBUG [7c96ac77] 	rake aborted!
DEBUG [7c96ac77] 	You have already activated rake 0.9.6, but your Gemfile requires rake 10.1.0. Using bundle exec may solve this.
DEBUG [7c96ac77] 	/home/deploy/navigator/releases/20130805155434/config/boot.rb:4:in `<top (required)>'
DEBUG [7c96ac77] 	/home/deploy/navigator/releases/20130805155434/config/application.rb:1:in `<top (required)>'
DEBUG [7c96ac77] 	/home/deploy/navigator/releases/20130805155434/Rakefile:4:in `<top (required)>'
DEBUG [7c96ac77] 	(See full trace by running task with --trace)
```

I could also override `:rake` in `SSHKit.config.command_map`, but it's been already used to rbenv integration.