Fix signal type error in CompletionView.

On Travis CI we are sometimes seeing:

```
CompletionView.selection_changed[str].emit():
argument 1 has unexpected type 'int'
```

Cast the data to a string before emitting it just to be safe.

qutebrowser/completion/completionwidget.py

@@ -344,7 +344,7 @@ class CompletionView(QTreeView):
         indexes = selected.indexes()
         if not indexes:
             return
-        data = self.model().data(indexes[0])
+        data = str(self.model().data(indexes[0]))
         self.selection_changed.emit(data)
 
     def resizeEvent(self, e):