Support DHCP IP addressing for Vagrant

I patched `get_environment_ips` to get IP address assigned by DHCP as well.

My code looks ugly a bit, but I hope it would be helpful.

I tested it with Vagrant v1.6.0 and VirtualBox v4.3.10 on Mac OSX 10.9.2, and Parallels Desktop 9 with vagrant-parallels plugin v1.8.0, too.