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.
`grep session.save_path #{ini_file_path} | cut -d= -f2 | sudo xargs mkdir -p` is actually shorter by a couple of characters if we are going to golf it and has the same output as the awk solution.