Properly fix issue with checking for a newer file.
Issue was that the hash mismatch checked the hash based off the *current* seek position, but the hash was from the last time it was saved (which might not be exactly where the seek position is). Therefore we should update the hash anytime we compare for a newer file.
Merge pull request #711 from mozilla-services/features/fix-rotation-check

Properly fix issue with checking for a newer file.