fix sync --dry-run traceback, bug #124

Bug 124: https://github.com/s3tools/s3cmd/issues/124

s3cmd sync --dry-run --recursive BLAH/ s3://blah-blah-blah
would traceback:

File "/usr/local/bin/s3cmd", line 1075, in _child
    output(u"remote copy: %s -> %s" % (dst1['object_key'],
    remote_list[dst2]['object_key']))
TypeError: string indices must be integers

In this case, dst1 and dst2, coming from copy_pairs, are themselves
the strings we want, not a lookup into a list.  So use them directly.