upgrade rails to 4.1.8

to address [CVE-2014-7829](https://groups.google.com/forum/#!topic/rubyonrails-security/rMTQy4oRCGk)
Merge pull request #152 from bitzesty/upgrade-rails

upgrade rails to 4.1.8
Add CHANGELOG since last release
Use released version of slimmer (5.0.1)
Merge pull request #153 from alphagov/slimmer-5.0.1

Use released version of slimmer (5.0.1)
update changelog
update change log with correct release
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
use ruby 2.1.4
fixing broken jquery plugins by implementing $.browser in newer jquery version
Update rbenv and ruby-build definitions

So that containers are able to install ruby 2.1.4
we don't need apt-get