Improve network performance for VirtualBox

Without those lines, VitrualBox machines are being very slow with all network operations (tested only in OS X Lion, but they are supposed to work everywhere). See https://github.com/mitchellh/vagrant/issues/1807
testing, issue #1766: Add nightly release to docker-ci
testing, issue #1766: Upgrade tooling for Docker nightly release
testing infrastructure, issue #1766: Temporarily install docker with proper apparmor handling
testing infrastructure, issue #1766: Dockerize deployment of docker-ci
Add vagrant user to the docker group
Merge pull request #2178 from tianon/vagrant-docker-group

Add vagrant user to the docker group
Merge pull request #1813 from doubleyou/patch-1

Improve network performance for VirtualBox
Merge pull request #1823 from dotcloud/1766-nightly-release

testing, issue #1766: Add nightly release to docker-ci