Update dvm to use boot2docker's $EXTRA_ARGS config option

boot2docker added the `EXTRA_ARGS` config option not too long ago, this pull request updates the `Vagrantfile` to use this instead of `sed`ing the init script for docker. 

Tested with the same `.kitchen.yml` as PR #21 ([link](https://gist.github.com/yacn/9195212#file-kitchen-yml))