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
"        }\n"
            "       \"final_scriptwitness\": [\"hex\", ...] (array of string) hex-encoded witness data (if any)\n"
            "      \"unknown\" : {                (json object) The unknown global fields\n"
            "        \"key\" : \"value\"            (key-value pair) An unknown key-value pair\n"
            "         ...\n"
            "      },\n"
            "    }\n"
            "    ,...\n"
            "  ]\n"
            "  \"outputs\" : [                 (array of json objects)\n"
            "    {\n"
            "      \"redeem_script\" : {       (json object, optional)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"
            "          \"hex\" : \"hex\",            (string) The hex\n"
            "          \"type\" : \"pubkeyhash\",    (string) The type, eg 'pubkeyhash'\n"
            "        }\n"
            "      \"witness_script\" : {       (json object, optional)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"
            "          \"hex\" : \"hex\",            (string) The hex\n"
            "          \"type\" : \"pubkeyhash\",    (string) The type, eg 'pubkeyhash'\n"
            "      }\n"
            "      \"bip32_derivs\" : [          (array of json objects, optional)\n"
            "        {\n"
            "          \"pubkey\" : \"pubkey\",                     (string) The public key this path corresponds to\n"
            "          \"master_fingerprint\" : \"fingerprint\"     (string) The fingerprint of the master key\n"
            "          \"path\" : \"path\",                         (string) The path\n"
            "          }\n"
            "        }\n"
            "        ,...\n"
            "      ],\n"