Ensure that HHVM defined session.save_path exists

Work around hhvm/packaging#81 by ensuring that any session.save_path
defined in /etc/hhvm/php.ini is created on the local file system. This
change is intended to fix travis-ci/travis-ci#2942.
Merge pull request #327 from bd808/issue/2942

Ensure that HHVM defined session.save_path exists
Add spec for https://github.com/travis-ci/travis-build/pull/327