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
{ "blockchain",         "getblockcount",          &getblockcount,          {} },
    { "blockchain",         "getblock",               &getblock,               {"blockhash","verbosity|verbose"} },
    { "blockchain",         "getblockhash",           &getblockhash,           {"height"} },
    { "blockchain",         "getblockheader",         &getblockheader,         {"blockhash","verbose"} },
    { "blockchain",         "getchaintips",           &getchaintips,           {} },
    { "blockchain",         "getdifficulty",          &getdifficulty,          {} },
    { "blockchain",         "getmempoolancestors",    &getmempoolancestors,    {"txid","verbose"} },
    { "blockchain",         "getmempooldescendants",  &getmempooldescendants,  {"txid","verbose"} },
    { "blockchain",         "getmempoolentry",        &getmempoolentry,        {"txid"} },
    { "blockchain",         "getmempoolinfo",         &getmempoolinfo,         {} },
    { "blockchain",         "getrawmempool",          &getrawmempool,          {"verbose"} },
    { "blockchain",         "gettxout",               &gettxout,               {"txid","n","include_mempool"} },
    { "blockchain",         "gettxoutsetinfo",        &gettxoutsetinfo,        {} },
    { "blockchain",         "pruneblockchain",        &pruneblockchain,        {"height"} },
    { "blockchain",         "savemempool",            &savemempool,            {} },
    { "blockchain",         "verifychain",            &verifychain,            {"checklevel","nblocks"} },
    { "blockchain",         "preciousblock",          &preciousblock,          {"blockhash"} },
    
    { "hidden",             "invalidateblock",        &invalidateblock,        {"blockhash"} },
    { "hidden",             "reconsiderblock",        &reconsiderblock,        {"blockhash"} },
    { "hidden",             "waitfornewblock",        &waitfornewblock,        {"timeout"} },
    { "hidden",             "waitforblock",           &waitforblock,           {"blockhash","timeout"} },
    { "hidden",             "waitforblockheight",     &waitforblockheight,     {"height","timeout"} },
};
void RegisterBlockchainRPCCommands(CRPCTable &t)
{
    for (unsigned int vcidx = 0; vcidx < ARRAYLEN(commands); vcidx++)
        t.appendCommand(commands[vcidx].name, &commands[vcidx]);
}