use official Docker ruby image with 2.1.2 tag

@aalness @adrianmacneil 

The Ruby image specified in `Dockerfile` no longer exists on Dockerhub. This causes `sudo docker build` to fail:
```shell
~/toshi$ sudo docker build -t=coinbase/node .
Sending build context to Docker daemon 13.03 MB
Sending build context to Docker daemon
Step 0 : FROM litaio/ruby:2.1.2
Pulling repository litaio/ruby
2014/12/09 06:09:50 Tag 2.1.2 not found in repository litaio/ruby
```

The only tag for `litaio/ruby` is `2.1.5`.
https://registry.hub.docker.com/u/litaio/ruby/

It seems that Docker now has official base images for Ruby
https://registry.hub.docker.com/_/ruby/

So, in this PR I've changed the base Ruby image to the official one from Docker. Thoughts?