Vagrant version specific logging (Fixes #41)

Vagrant changed it's logging API in version 1.5. This pull request fixes issue #41 by using the correct logging API based on the version of Vagrant that is loaded. The source from this was copied from https://github.com/sneal/vagrant-butcher/commit/80c87eaef0fa8ab17362b9eaad41033dd2cccbe7