address_types.py test seems to fail intermittently on travis

It was mentioned in https://github.com/bitcoin/bitcoin/pull/11991#issuecomment-357922664 that this could be due to sync_mempools failing. A failure I saw looked like:

```
address_types.py failed, Duration: 14 s
stdout:
2018-01-12 23:14:39.513000 TestFramework (INFO): Initializing test directory /tmp/bitcoin_test_runner_20180112_231145/address_types_178
2018-01-12 23:14:48.747000 TestFramework (INFO): Sending from node 0 (['-addresstype=legacy']) without multisig using default
2018-01-12 23:14:51.012000 TestFramework (INFO): Sending from node 1 (['-addresstype=p2sh-segwit']) without multisig using default
2018-01-12 23:14:53.263000 TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/test_framework.py", line 123, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/address_types.py", line 183, in run_test
    assert_equal(unconf_balances[to_node], to_send * 10 * (2 + n))
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/util.py", line 38, in assert_equal
    raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
AssertionError: not(0E-8 == 1176.25722960)
2018-01-12 23:14:53.263000 TestFramework (INFO): Stopping nodes
2018-01-12 23:14:53.577000 TestFramework (WARNING): Not cleaning up dir /tmp/bitcoin_test_runner_20180112_231145/address_types_178
2018-01-12 23:14:53.577000 TestFramework (ERROR): Test failed. Test logging available at /tmp/bitcoin_test_runner_20180112_231145/address_types_178/test_framework.log
2018-01-12 23:14:53.578000 TestFramework (ERROR): Hint: Call /home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/combine_logs.py '/tmp/bitcoin_test_runner_20180112_231145/address_types_178' to consolidate all logs
```
https://travis-ci.org/bitcoin/bitcoin/jobs/328318030
obj.push_back(Pair("verificationprogress",  GuessVerificationProgress(Params().TxData(), chainActive.Tip())));
    obj.push_back(Pair("initialblockdownload",  IsInitialBlockDownload()));
    obj.push_back(Pair("chainwork",             chainActive.Tip()->nChainWork.GetHex()));
    obj.push_back(Pair("size_on_disk",          CalculateCurrentUsage()));
    obj.push_back(Pair("pruned",                fPruneMode));
    if (fPruneMode) {
        CBlockIndex* block = chainActive.Tip();
        assert(block);
        while (block->pprev && (block->pprev->nStatus & BLOCK_HAVE_DATA)) {
            block = block->pprev;
        }
        obj.push_back(Pair("pruneheight",        block->nHeight));
        
        bool automatic_pruning = (gArgs.GetArg("-prune", 0) != 1);
        obj.push_back(Pair("automatic_pruning",  automatic_pruning));
        if (automatic_pruning) {
            obj.push_back(Pair("prune_target_size",  nPruneTarget));
        }
    }
    const Consensus::Params& consensusParams = Params().GetConsensus();
    CBlockIndex* tip = chainActive.Tip();
    UniValue softforks(UniValue::VARR);
    UniValue bip9_softforks(UniValue::VOBJ);
    softforks.push_back(SoftForkDesc("bip34", 2, tip, consensusParams));
    softforks.push_back(SoftForkDesc("bip66", 3, tip, consensusParams));
    softforks.push_back(SoftForkDesc("bip65", 4, tip, consensusParams));
    for (int pos = Consensus::DEPLOYMENT_CSV; pos != Consensus::MAX_VERSION_BITS_DEPLOYMENTS; ++pos) {
        BIP9SoftForkDescPushBack(bip9_softforks, consensusParams, static_cast<Consensus::DeploymentPos>(pos));
    }
    obj.push_back(Pair("softforks",             softforks));