Use .galaxy_install_info to trigger role installation

The `ansible-galaxy` command is nice enough to leave behind a bit of metadata associated with role installation (`role_name/meta/.galaxy_install_info`). The `Vagrantfile` now attempts to determine the version of an installed role based off of the metadata in that file. If the versions don't match, an install of all role dependencies occurs.

I'm not in love with this solution, but I think that it is better than force reinstalling all role dependencies every time (~10 seconds on every provision).
Is it guaranteed that the ``meta/.galaxy_install_info`` file will always exist if the ``role_path`` directory exists? If not, I would suspect that this would throw a file not found exception in that case (although I don't know the specifics of what the YAML library does).