BUG: correct next_revision, previous_revision methods

`Revision.next_revision` and `Revision.previous_revision` does not work correctly.

The methods implementation is particularly reverted as in previous

Bug raised at: 35f3395c2eaf149e4d2392606a24ac237a753c26
Previous implementation can be found at: d9f6a859d726b164e584047c3436d89c2ae54c79
Merge pull request #161 from ksurya/fix-pag

BUG: correct next_revision, previous_revision methods