Fix bundler version error
Fix bundler version error

Vagrant relies on some internal APIs in Bundler core.
The command of `bundle exec itamae ssh --vagrant` will conflict with Vagrant APIs.

Using `Bundler.with_clean_env` is necessary when using itamae with bundler.

* ref:
  * #185
  * https://github.com/mitchellh/vagrant/issues/5581#issuecomment-91889166

itamae-kitchen/itamae,uroesch/itamae,itamae-kitchen/itamae,sue445/itamae,uroesch/itamae,sue445/itamae,maruware/itamae,maruware/itamae