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.
Perhaps this is a minor point, but you can reduce the clutter into a single `awk` command:

`awk -F= '/session.save_path/ {print $2}' #{ini_file_path} | sudo xargs mkdir -p`