IE8 JS: Fix GOVUK.cookie not being defined

See: https://github.com/alphagov/static/pull/380 for related discussion
Switch to errbit

Needs staging, preview, production configs to be set
Merge pull request #122 from alphagov/errbit

Switch to errbit
Update airbrake
Merge pull request #124 from alphagov/airbreak

Update airbrake
Reduce crawler load

> Look to set appropriate entry in robots.txt to stop crawlers hammering the site.
> Since we allow searching back across time, we can let bots look at lots of content that is otherwise not used (and isn't in caches). This can be quite expensive.

- Add `rel='nofollow'` on the links that do the filtering
- Add `X-Robots-Tag` header on historical pages

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68707106)
Merge pull request #125 from bitzesty/reduce-crawler-load

Reduce crawler load
do not show section duplicates when searching via synonyms

> If we assign two synonyms to the same location (heading, commodity) when we search the result appears twice.
> We should dedup the response, so we only show one result, per location.

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68999382)
Use `X-Robots-Tag` header in controllers that inherit from `ApplicationController`

@matthewford's comment on PT:
> @macool The robots meta works in the search, but also needs to be added in the other controllers for headings, sections, commodities too.

- Add `X-Robots-Tag` header on historical pages
- Added a spec for `SectionsController` related to `X-Robots-Tag`

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68707106)
Merge pull request #127 from bitzesty/reduce-crawler-load

Use `X-Robots-Tag` header in controllers that inherit from `ApplicationController`
remove de-duplication logic

This should be handled by backend. Added in alphagov/trade-tariff-backend#159
Merge pull request #126 from bitzesty/dedup-synonyms

do not show section duplicates when searching via synonyms
use `faraday` instead of `httparty`

[pivotal story](https://www.pivotaltracker.com/story/show/69001432)
use `faraday_middleware`

So that it parses responses as json instead of doing it manually
Merge pull request #128 from bitzesty/faraday

Use faraday instead of httparty
add `tariff:diff` task

Usage:
`rake tariff:diff[host1,host2] chapter=01`
It traverses the tree beginning on the specified chapter and compares all nodes between two endpoints

If the host needs basic authentication, you can set username and password on `host1user` and `host1passwd` or `host2user` and `host2passwd`
Example:
`rake tariff:diff[https://www.gov.uk/,http://tariff.dev.gov.uk/] host2user=macool host2passwd=marioandres chapter=01`

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/69914024)
write `TariffDiff` output to rails logger

And mute it on specs
Merge pull request #129 from bitzesty/69914024-diff-script

tariff:diff rake task for comparing data on two hosts
Add guidance for contributors
Upgrade to Ruby 2.0.0

Also pin yajl-ruby to 1.2.0