Auto merge of #729 - aneeshusa:upgrade-to-vagrant-2, r=larsbergstrom
Auto merge of #729 - aneeshusa:upgrade-to-vagrant-2, r=larsbergstrom

Upgrade to Vagrant 2 for `salt_call_args`

Vagrant 2.0.0 adds support for a new `salt_call_args` option.
We use this option to specify some additional arguments for Vagrant.

Previously, we used a hack to pass these options via the `minion_id`
parameter, but that hack stopped working as of Vagrant 1.8.3

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/saltfs/729)
<!-- Reviewable:end -->

servo/saltfs,servo/saltfs