sync remote2remote: set target_uri on updated files too, bug 131

fixes bug https://github.com/s3tools/s3cmd/issues/131

We were not setting the target_uri field on files that had changed,
only on files that were to be newly uploaded.  This was causing a
traceback when referencing the target_uri field when copying the
changed file.
Merge pull request #133 from mdomsch/bug/131

sync remote2remote: set target_uri on updated files too, bug 131