Use bundler for gem management

As requested in #117, this PR uses [bundler](http://bundler.io/) to manage the ruby gems we use for Jekyll and its various and sundry plugins. Using Bundler instead of Ansible's `gem` module has the advantage that it also locks the versions of all transitive dependencies of the gems we use, until we consciously upgrade them, so it enforces further consistency in our development environments.

Bundler is idempotent on its own -- if all of the necessary gems are already installed, it will do nothing.