Add DigitalOcean driver for Test Kitchen

We’ll still use Vagrant by default, but give ourselves the option to
set an alternative driver via .kitchen.local.yml
Add build-essential to mongo_gem recipe

Build tools may not be installed on the host by default, making the
bson_ext native gem installation fail.
Update dependency versions
Don't run build-essentials at compile time

Otherwise it will try to run before the “apt” recipe had a chance to
update the repositories on the first run, and will fail on some of the
packages.
Fix for slow Vagrant networking issue
Update to latest version of MongoDB and Ruby driver
Make user provider more resilient

Setting permissions for a replica set on the initial chef run may
sometimes fail, probably due to the replica set still being in an
inconsistent (initial) state - despite the cluster reporting that it’s
ready. Utilising the same retry mechanism we have for node and replica
set state checks seems to solve the problem.
Update compatibility and version numbers in Readme
Changelog for 1.2.0 release