Remove "how to build" documentation

Instead, point to the CI configuration files, and include the extra
steps for Ubuntu 12.04 as a script. More useful, and less likely to
become outdated.

scripts/linux/prepare_ubuntu_precise.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+# This script prepares a fresh install of Ubuntu Server 12.04 so that the steps
+# in .travis.yml can then be run.
+
+sudo rm -rf /var/lib/apt/lists/*
+sudo apt-get update
+
+sudo apt-get install -y python-software-properties git build-essential libcurl4-openssl-dev
+
+sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 68576280
+sudo apt-add-repository -y 'deb https://deb.nodesource.com/node_4.x precise main'
+sudo apt-get update
+sudo apt-get install -y nodejs