uses bundled berks and not system-wide berk + fixed small typo (berksintall)

=> avoids conflicts with berks v3
packer 0.6.0 wants a default region

“Build 'digitalocean' errored: Error creating droplet: Received error
from DigitalOcean (404)
: You specified an invalid region for Droplet creation.”
Merge pull request #1 from sjourdan/fix_berks

Thanks for the PR - merging.