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