Fix getLatestRevisionId return value in WikiPageEntityLookup

This returning a string instead of int, causing comparisons
like  $entityRevision->getRevision() !== $revisionId in
CachingEntityRevisionLookup thus never work and then the
code fallsback to database lookup instead of utilizing
the caching mechanism.

Change-Id: I60c6ef4e385cb00db664471d250c80b66b2a3aad