Fix bootstrap.sh with newer bundler

This will avoid error:
```bash
==> default: /usr/lib/ruby/2.0.0/fileutils.rb:245:in `mkdir'
==> default: : 
==> default: Permission denied - /root/.bundler
==> default:  (
==> default: Errno::EACCES
==> default: )
==> default: 	from /usr/lib/ruby/2.0.0/fileutils.rb:245:in `fu_mkdir'
```