Fix misc test failures stream and non-stream builds

Thanks for the clarification. I think there were other differences between 2.6 and 2.4, which is why I ended up bumping this to `>= 2.5.5`.

FWIW, I also ran the test suite against 2.2.7 in both stream/non-stream builds and we only had two failures:

 * MongoClient::__construct() during failover (1) [tests/replicaset-failover/mongoclient-construct-001.phpt]
 * Test for PHP-1048: Segfault killing cursors after failover [tests/replicaset-failover/mongoinsertbatch-execute-002.phpt]

Both were due to the following exception message difference:

```
+ string(41) "localhost:30200: Connection reset by peer"
- string(%d) "%s:%d: Remote server has closed the connection"
```