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
{
    if (request.fHelp || request.params.size() != 0)
        throw std::runtime_error(
            "getblockchaininfo\n"
            "Returns an object containing various state info regarding blockchain processing.\n"
            "\nResult:\n"
            "{\n"
            "  \"chain\": \"xxxx\",              (string) current network name as defined in BIP70 (main, test, regtest)\n"
            "  \"blocks\": xxxxxx,             (numeric) the current number of blocks processed in the server\n"
            "  \"headers\": xxxxxx,            (numeric) the current number of headers we have validated\n"
            "  \"bestblockhash\": \"...\",       (string) the hash of the currently best block\n"
            "  \"difficulty\": xxxxxx,         (numeric) the current difficulty\n"
            "  \"mediantime\": xxxxxx,         (numeric) median time for the current best block\n"
            "  \"verificationprogress\": xxxx, (numeric) estimate of verification progress [0..1]\n"
            "  \"initialblockdownload\": xxxx, (bool) (debug information) estimate of whether this node is in Initial Block Download mode.\n"
            "  \"chainwork\": \"xxxx\"           (string) total amount of work in active chain, in hexadecimal\n"
            "  \"size_on_disk\": xxxxxx,       (numeric) the estimated size of the block and undo files on disk\n"
            "  \"pruned\": xx,                 (boolean) if the blocks are subject to pruning\n"
            "  \"pruneheight\": xxxxxx,        (numeric) lowest-height complete block stored (only present if pruning is enabled)\n"
            "  \"automatic_pruning\": xx,      (boolean) whether automatic pruning is enabled (only present if pruning is enabled)\n"
            "  \"prune_target_size\": xxxxxx,  (numeric) the target size used by pruning (only present if automatic pruning is enabled)\n"
            "  \"softforks\": [                (array) status of softforks in progress\n"
            "     {\n"
            "        \"id\": \"xxxx\",           (string) name of softfork\n"
            "        \"version\": xx,          (numeric) block version\n"
            "        \"reject\": {             (object) progress toward rejecting pre-softfork blocks\n"
            "           \"status\": xx,        (boolean) true if threshold reached\n"
            "        },\n"
            "     }, ...\n"
            "  ],\n"