Block ahrefs bot in robots.txt

This bot crawls us very very regularly (sometimes more than once a day)
and causes spikes in our load and exceptions. It seems to serve no useful
purpose to us, in that it's not a search engine that users would use.

 > 1. What is ahrefs.com
 > Ahrefs.com is an independent tool for SEO analysis with a wide range
 > of features. It is designed, first of all, for SEO specialists and
 > site owners but may be of interest to other concerned Internet
 > researchers.
 > https://ahrefs.com/faqs.php

This change to the robots.txt is the recommended way to stop the bot
crawling. See https://ahrefs.com/robot
Merge pull request #408 from alphagov/block-ahrefs

Block ahrefs bot in robots.txt
Update jasmine to 2.0.2

Also cleaned up the jasmine environment. Jasmine 2
uses poltergeist instead of selenium-webdriver and
so is jenkins-friendly. Additionally, it's now 
possible to use the asset pipeline to include source
assets instead of having to maintain a list in jasmine.yml.

Tests do not pass (and didn't before this change).
Added jquery to jasmine manifest
Stop Jasmine auto-installing phantom js
Repair report-a-problem-spec.js

Also adds jasmine-jquery and renames spec file to
match source file.
Repaired events-cookie-handler-spec (was AnalyticsTokenSpec)
Move InternalSiteEventsSpec into events-cookie-handler-spec

And repair it.
Repair tracking-spec (was SuccessEventTrackingSpec)
Fix user-satisfaction-survey-spec
Fix tests in phantom.

When using phantomJS JSON.parse seems bugged, returning a string
instead of a built object. I'm unsure as to why, since when testing
this behaviour directly in phantom it behaves as expected. This
monkey patches the issue, but It'd be good to find out what's
going on here.
Added note on running JS tests to readme
Jasmine runs as part of default rake task
Modified wrapping on some JS specs
Added contentSeenTracker script

This script looks for elements with the attribute 'data-track-seen' and
sends an analytics event when the user scrolls the page to show that element.

An addition will be made to govspeak to allow editors to add these tracker
tags to content.