Conteinarize snowcrash environment using Docker

Few notes:

* The `libgemplugin-ruby` is needed to make `gem` command available
* It would be better to move `provisioning.sh` directly into `Dockerfile` because of caches, but wanted to keep things DRY. The "proper DRY" solution would be to run docker inside Vagrant, but don't want to impose that

* It might be worth uploading the image to DockerHub when snowcrash is released. Do that with `docker build apiaryio/snowcrash:$version . && docker push apiaryio/snowcrash:$version`.