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
Bump govuk_template to 0.8.1 to get the proposition header changes, for Whitehall
Merge pull request #411 from alphagov/update-template-version

Bump govuk_template to 0.8.1 to get Proposition Header changes
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