Skip new SRV test if dnspython isn't available

After merging https://github.com/mongodb/mongo-python-driver/commit/a23ce28942c8e404e6a0cadb7dccc24567851ce3 test failures are being seen on TravisCI - https://travis-ci.org/mongodb/mongo-python-driver/jobs/559665979. This is due to the fact that the newly added test assumes dnspython is installed which it isn't on Travis.