Make travis validate json

bug fix to PR #21: fixed variable names not committed

As of writing there is no 'latest' (hashicorp/packer/issues/5265)

I'm sorry, I didn't double check what's staged and thus didn't commit fixed
variable names: 'iso_checksum' -> 'iso_checksum_url'

replace iso_checksum with iso_checksum_url

By replacing iso_checksum with iso_checksum_url there is no need to
hardcode hash values into vagrant.json.

Trying to add mirror variable leeds to errors: vars inside variables are not
reusable. See packer issue (#874)[https://github.com/hashicorp/packer/issues/874]

We need a sequence if we want to use the vagrant-cloud post-processor.

Write zeros to disk to improve disk compression

Write zeros to a file on the root partition until the filesystem is
full, then unlink the file. Packer will compress the disk as part of
the build, resulting in smaller final disk image.

I also enabled headless for future headless builds on a buildserver

Vmware support

Fix the building proccess

Remove header/dkms (Closes #4)

Setting up authorization rule for Polkit

Use Virtualbox guest kernel modules for Arch Kernel (virtualbox-guest-modules-arch).

This commit fixes the 'file not found: `fsck.btrfs''-Error during
system upgrade.