Provide a box url in the Vagrantfile

this allows users who don't already have a Vagrant box called
ubuntu/precise64 to build a Vagrant VM.
Install prerequisites for the chruby installation
set -e in inline provisioning scripts

This causes errors in the script to immediately abort processing, which
makes debugging much easier.
Update the vagrantfile to ruby 2.1.2

2.1.2 has been required by the Gemfile since
06cc2ef6ca4fb0a2ba38f3eb422b036916f44e97
Merge pull request #291 from AndrewLorente/vagrantfile-fixes

Fix the Vagrantfile