[SP-1090] Version dates do not change

Backport of Regression [PDI-11359]
Timestamp for jobs in DI repository continuously displays the same value for different versions.
Fixed by creating new timestamp at check-in of JCR File.

http://jira.pentaho.com/browse/SP-1090
Merge pull request #1236 from hudak/SP-1090

[SP-1090] Version dates do not change