Fixing excessive sudo'ing

[zorbathut]

Hey there! I recently used this guide to get Vagrant working on Travis. As part of this, I ran into some problems that I thought were caused by excessive sudo'ing for Vagrant. It later turned out they weren't, but as part of fixing this, I discovered how to get rid of the unnecessary sudo vagrant.

My final file is located at https://github.com/themotte/tildes/blob/master/.travis.yml but the tl;dr is:

• libvirt by default requires the user to be under the libvirt group, but there doesn't seem to be any working way to change the Travis user's group at runtime; use sed to edit libvirt's config file to refer to the travis group instead, then restart libvirt
• sudo vagrant plugin install causes plugins to be installed as root, which causes access issues; remove the sudo to fix it

I'm not sure how much you care about not running vagrant as root, but it'd probably make things easier for other people. I can put together an actual pull request if you'd like!

[jonashackt]

Sounds good to me, would be great to see a PR :)