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{"decodepsbt",
                "\nReturn a JSON object representing the serialized, base64-encoded partially signed Bitcoin transaction.\n",
                {
                    {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "The PSBT base64 string"},
                },
                RPCResult{
            "{\n"
            "  \"tx\" : {                   (json object) The decoded network-serialized unsigned transaction.\n"
            "    ...                                      The layout is the same as the output of decoderawtransaction.\n"
            "  },\n"
            "  \"unknown\" : {                (json object) The unknown global fields\n"
            "    \"key\" : \"value\"            (key-value pair) An unknown key-value pair\n"
            "     ...\n"
            "  },\n"
            "  \"inputs\" : [                 (array of json objects)\n"
            "    {\n"
            "      \"non_witness_utxo\" : {   (json object, optional) Decoded network transaction for non-witness UTXOs\n"
            "        ...\n"
            "      },\n"
            "      \"witness_utxo\" : {            (json object, optional) Transaction output for witness UTXOs\n"
            "        \"amount\" : x.xxx,           (numeric) The value in " + CURRENCY_UNIT + "\n"
            "        \"scriptPubKey\" : {          (json object)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"