Fix plugin to work with vagrant 1.4.0+

Vagrant 1.4.0+ doesn't always have env[:home_path] set. I didn't try to find out why. This fix is a bit of a hack, since it just defaults to the .vagrant.d directory in the user's home. As for the ui object, it looks like it changed from a hash variable to an instance variable of the environment class.

We tested this in our project team with Vagrant 1.3.5 and 1.4.3 on Ubuntu 12.10, and plugin works fine for us.