Evaluate usage of CronExpression

[coreymckrill]

As noted here, we are shipping an old, abandoned version of CronExpression in the lib folder. The library has since been forked by a new maintainer, and continues to be maintained to this day. Keeping around the old version is risky for PHP compatibility and security reasons, so I think we should:

• Evaluate whether or not we still need to use this library
• If so, consider removing it from version control and instead adding it as a composer dependency. This would also require auditing our usage of it, as the latest version has some breaking changes from the version we're currently using.

[barryhughes]

Updating it or replacing it may be safest from a back-compat standpoint, so long as we can maintain the same interface for our users. Support for cron expressions does feel like a situation where we're offering more than we perhaps should, but, it would also be hard to assess the impact of dropping support.