Better nModified handling PYTHON-647

In a sharded cluster with mixed version shards (i.e.
one shard primary is 2.6, another is 2.4 or older) the
result of one call to update could include nModified
when the next does not. Since there is no way to provide
a valid count in this case we omit the field. Make sure
you upgrade your entire cluster to MongoDB 2.6 or newer
before relying on this field.