In the require or alternatively in the require-dev section, add the following dependency:
with $VERSION being one of the versions available at Packagist.
If the PEAR installer is available on your system, you only need to execute the following commands:
$ pear channel-discover pear.bovigo.org $ pear install bovigo/vfsStream-beta
If you already registered the pear.bovigo.org channel before you can skip the first command.