Use Ruby instead of Perl to parse synced_folders in Vagrantfile

[westonruter]

We're currently using regular expressions to parse the Vagrantfile: https://github.com/x-team/vassh/blob/665b71f8540653579035e007c38da3f86d0c5d24/vassh.sh#L52-L80

This is brittle. The Vagrantfile is just Ruby code, and we should be using Ruby to parse it. This will also help us detect and account for multi-machine Vagrantfiles.