Modify policy of option --max-candidates so that it actually works

Select the top max candidates ignoring redundant or invalid candidates
without race condition (indeterminism).

This was the previous behavior before I replaced it with something
that would not account for redundant or invalid candidates, except it
is deterministic.

So this potentially modifies the behavior of MOSES, you might want to
lower --max-candidates to get back to similar behavior in term of
number of candidates merged.
Set moses version to 3.6.7 because the behavior has slightly changed
Merge pull request #203 from ngeiswei/max_pot_cnd

Max pot cnd