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
},
                RPCExamples{
                    HelpExampleCli("decoderawtransaction", "\"hexstring\"")
            + HelpExampleRpc("decoderawtransaction", "\"hexstring\"")
                },
    };
    if (request.fHelp || !help.IsValidNumArgs(request.params.size())) {
        throw std::runtime_error(help.ToString());
    }
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VBOOL});
    CMutableTransaction mtx;
    bool try_witness = request.params[1].isNull() ? true : request.params[1].get_bool();
    bool try_no_witness = request.params[1].isNull() ? true : !request.params[1].get_bool();
    if (!DecodeHexTx(mtx, request.params[0].get_str(), try_no_witness, try_witness)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    }
    UniValue result(UniValue::VOBJ);
    TxToUniv(CTransaction(std::move(mtx)), uint256(), result, false);
    return result;
}
static std::string GetAllOutputTypes()
{
    std::string ret;
    for (int i = TX_NONSTANDARD; i <= TX_WITNESS_UNKNOWN; ++i) {
        if (i != TX_NONSTANDARD) ret += ", ";
        ret += GetTxnOutputType(static_cast<txnouttype>(i));
    }
    return ret;
}
static UniValue decodescript(const JSONRPCRequest& request)