wrong handling of multiple unicode literals when --six-unicode is used

If one has:

```
    foo(u'.........'
        u'.........')
```

it gets converted to:

```
    foo(six.u('........')
        six.u('........'))
```

that obviously does not work, better would be:

```
    foo(six.u('.........'
              '.........'))
```

(moved from https://github.com/mitsuhiko/python-modernize/issues/9 opened by @ThomasWaldmann)
