upgrade to rails 4

also some amendments to specs to make them work
force #to_s when parameterizing Section
use `xvfb` to run tests on travis
Revert "use `xvfb` to run tests on travis"

This reverts commit 435406dd20506677c77c6897a4684d66e8134944.
Merge pull request #145 from bitzesty/rails-4

upgrade to rails 4
require active_model in tariff_date
Run request_forwarder_specs (were being ignored)
Remove pending specs as Rails 4 raises ActionController::UnknownFormat
Fix pending spec as around needs an argument
Fix SearchReferencesController specs to run

Missing around argument
Merge pull request #146 from alphagov/missing-require

Fix specs
Update Readme with link to backend application
Add license title and update copyright year
Update env configs, plek & slimmer

Removes commented out config options only applicable to Rails 3

In production:
use plek to set asset_host
specify js_compressor

Slimmer version which loads assets from production
Plek for asset_root
Merge pull request #147 from alphagov/assets-fix

Update env configs, plek & slimmer
change licence to markdown
Update README.md
Configure poltergeist to allow any SSL protocol

This might help with the loading of assets.
Also makes the version of poltergeist explicit