Fixed PHP-1257: Support 50 replicaset members

Bump our max ReplicaSet member count to 64 to support MongoDB 2.8 (50nodes).

This is just a stop-gap messurement until we can figureout a solution
for supporting infinate secondaries.
Merge Pull Request #749

* PHP-1257-50replset:
  Fixed PHP-1257: Support 50 replicaset members