Ensure Vagrant version is at least 1.8.0

[aneeshusa]

Use the Vagrant.require_version helper documented here:
https://www.vagrantup.com/docs/vagrantfile/vagrant_version.html

Eliminates issues like #238.

Heads up: I didn't test this with a Vagrant older than 1.8.0, so I have no idea if this actually works.
@larsbergstrom can you test this if possible since you ran into this?
I want to make sure this is actually a useful change.

[edunham]

Build broke due to timeout issues so I've retried it through Travis

[edunham]

When I try this on Vagrant 1.4.3, I get the error:

Vagrant failed to initialize at a very early stage:

This Vagrant environment has specified that it requires the Vagrant
version to satisfy the following version requirements:

  >= 1.8.0

You are running Vagrant 1.4.3, which does not satisify
these requirements. Please change your Vagrant version or update
the Vagrantfile to allow this Vagrant version. However, be warned
that if the Vagrantfile has specified another version, it probably has
good reason to do so, and changing that may cause the environment to
not function properly.

In my opinion, forbidding Vagrant versions which sporadically break the build is a better use of our time than fixing each quirk that shows up only in old Vagrants.

r? @larsbergstrom

[larsbergstrom]

@bors-servo r+

[bors-servo]

📌 Commit 77a7a7a has been approved by larsbergstrom

[bors-servo]

⌛ Testing commit 77a7a7a with merge c1d73d7...

[bors-servo]

☀️ Test successful - travis