Migrate to XWiki 5.1

Need to migrate to XWiki 5.1 to overcome limitation on Job Module, that cannot run my custom job (because it gets only root component manager).

See http://xwiki.markmail.org/search/?q=#query:%20list%3Aorg.xwiki.users+page:2+mid:4gcp6lvathmu6j2n+state:results