Tighten up the cron regex

The old regex would allow cron-style strings such as '* * * * * * * *', which is
invalid. While it's very difficult, if not impossible, to regex a cron-style string
this should be a significant improvement.

It allows exactly 5 occurances of: any integer, comma, forward-slash or dash more than
once OR a single *.
Merge pull request #504 from dannyfallon/df/tighten-plain-cron-regex

Tighten up the cron regex