Load Bundler inside Rakefile

What do you think about requiring bundler in the beginning of `Rakefile` ? It will allow to run `rake` task without `bundle exec` prefix. 

Currently I have this backtrace after calling `rake -T`.
```
~/github/bosh [master] → rake -T
rake aborted!
cannot load such file -- bosh/dev/tasks
/home/lomov/work/github/bosh/Rakefile:3:in `<top (required)>'
(See full trace by running task with --trace)
``` 
This PR solves it.