Docker setup
docker enhances

- use `macool/baseimage-rbenv-docker:latest` as base image to build docker image (based on phusion's baseimage)
enhances to run with `tariff-docker`

- Add a `backend_ip.sh` initialiser to container so it adds backend development ip *tariff-api.dev.gov.uk* to `/etc/hosts`

For more info see [bitzesty/tariff-docker](https://github.com/bitzesty/tariff-docker)
fix image build
Upgrade rails & make it work with docker container
upgrading to rails 4
use sentry
add `docker` env

- use it by default on `startup.sh` (called by docker containers)
- copied from `production`, uses some caching and it's faster than `development`
- log this env's errors on sentry
fix Gemfile.lock
missing assets entry
use ruby 2.1.4