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 (psbtx.tx->nVersion > best_version) {
            best_version = psbtx.tx->nVersion;
        }
        
        if (psbtx.tx->nLockTime < best_locktime) {
            best_locktime = psbtx.tx->nLockTime;
        }
    }
    
    PartiallySignedTransaction merged_psbt;
    merged_psbt.tx = CMutableTransaction();
    merged_psbt.tx->nVersion = best_version;
    merged_psbt.tx->nLockTime = best_locktime;
    
    for (auto& psbt : psbtxs) {
        for (unsigned int i = 0; i < psbt.tx->vin.size(); ++i) {
            if (!merged_psbt.AddInput(psbt.tx->vin[i], psbt.inputs[i])) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Input %s:%d exists in multiple PSBTs", psbt.tx->vin[i].prevout.hash.ToString().c_str(), psbt.tx->vin[i].prevout.n));
            }
        }
        for (unsigned int i = 0; i < psbt.tx->vout.size(); ++i) {
            merged_psbt.AddOutput(psbt.tx->vout[i], psbt.outputs[i]);
        }
        merged_psbt.unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
    }
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << merged_psbt;
    return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
}
UniValue analyzepsbt(const JSONRPCRequest& request)