Fix cron validation for Jenkins>=1.558

[bjaglin]

See https://issues.jenkins-ci.org/browse/JENKINS-22550 for details about the exact same issue for the github plugin. Fix shamelessly copied from jenkinsci/ghprb-plugin#28

[timols]

Would you mind updating the Jenkins version in pom.xml?

[bjaglin]

Sure, I can. However, I am not familiar with Jenkins plugins development but since this change is supposedly backwards compatible, is there really a need to build against a very recent org.jenkins-ci.plugins:plugins parent?

[eddiemonge]

hope this can land soon. it prevents saving some configs unless the cron part is disabled (emptied out)

[theneva]

Huge +1.

[madsroskar]

+1!

[acr92]

+1

[Hellslicer]

+1

[arjes]

+1, I assume this will Close #56 that I opened.

[garbagetown]

+1

[muuki88]

+1

[timols]

I've merged in #62 which has the same changes but includes pom.xml changes.

[bjaglin]

@timols great! I'd still like to understand the rationale behind the bump though (and the reason why 1.532 was chosen) so I can do it properly next time.