travis run: https://travis-ci.org/bitcoin/bitcoin/jobs/406813397

```
rpc_psbt.py failed, Duration: 4 s
stdout:
2018-07-22T13:28:57.765000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_test_runner_20180722_132530/rpc_psbt_49
2018-07-22T13:29:00.531000Z 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 160, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/rpc_psbt.py", line 101, in run_test
    assert_raises_rpc_error(-22, "TX decode failed", self.nodes[0].converttopsbt, signedtx['hex'])
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/util.py", line 105, in assert_raises_rpc_error
    assert try_rpc(code, message, fun, *args, **kwds), "No exception raised"
AssertionError: No exception raised
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"combinerawtransaction",
                "\nCombine multiple partially signed transactions into one transaction.\n"
                "The combined transaction may be another partially signed transaction or a \n"
                "fully signed transaction.",
                {
                    {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of hex strings of partially signed transactions",
                        {
                            {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "A transaction hash"},
                        },
                        },
                },
                RPCResult{
            "\"hex\"            (string) The hex-encoded raw transaction with signature(s)\n"
                },
                RPCExamples{
                    HelpExampleCli("combinerawtransaction", "[\"myhex1\", \"myhex2\", \"myhex3\"]")
                },
            }.ToString());
    UniValue txs = request.params[0].get_array();
    std::vector<CMutableTransaction> txVariants(txs.size());
    for (unsigned int idx = 0; idx < txs.size(); idx++) {
        if (!DecodeHexTx(txVariants[idx], txs[idx].get_str(), true)) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed for tx %d", idx));
        }
    }
    if (txVariants.empty()) {