TED extractor: prefer 'code' to 'uri' if present

[minusf]

it seems like some TED videos pointing to Youtube define code,
and may or may not use uri for something else (pointing to the page of the actual TED event).

here is an actual example:
http://www.ted.com/talks/jeffrey_kluger_the_sibling_bond

{"talks": [{
        "id":1369,
        "duration":1268, 
        "canonical":"http://www.ted.com/talks/jeffrey_kluger_the_sibling_bond",
        "external": {
            "code":"aFBIPO-P7LM",
            "service":"YouTube",
            "uri":"http://www.ted.com/tedx/events/3601"
        },
    }]
}

[dstftw]

This will be fixed in the next version. Thanks for the report and PR.