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
re-add build script example artifacts
Redo https://github.com/travis-ci/travis-build/pull/298

While the corresponding cookbooks change should be deployed,
we continue to receive reports of
https://github.com/travis-ci/travis-ci/issues/2514.

The PR bit to travis-build was missed during the sexp refactoring,
so we redo it with this commit.
Merge pull request #348 from travis-ci/ha-bug-gh-2514-redux

Redo https://github.com/travis-ci/travis-build/pull/298
fix directory_cache to not raise if s3 config is missing
Merge pull request #350 from travis-ci/sf-cache-config-missing

Fix directory_cache to not raise if s3 config is missing
Merge branch 'master' into sf-restore-build-script-examples

Conflicts:
	spec/build/addons/firefox_spec.rb
Put back lost statement during the merge
Merge pull request #336 from travis-ci/sf-restore-build-script-examples

Build script example artifacts
Merge pull request #291 from AndrewLorente/vagrantfile-fixes

Fix the Vagrantfile
Revert "Drop '--jobs' from bundler args when running on Rubinius"

This reverts commit b64eea75aba4a42187aad8bff0be3e1702d379b5.

Rubinius 2.4.1 has bee released
(https://github.com/travis-ci/travis-ci/issues/2952#issuecomment-65842593)
and we would like to get some additional data if that release
is good for addressing the original problem.