make `VotesManager::offsetUnset` compatible
make `VotesManager::offsetUnset` compatible

`VotesManager::offsetUnset` has return type `void` while
`ArrayManager::offsetUnset` has return type `bool`. Gave
`VotesManager::offsetUnset` also the return type of `bool`
and return value of `true` when parent returned true, `false`
otherwise.

This should fix issue #15

julien-boudry/Condorcet