Write can't paste image copied from journal

[godiard]

TestCase

• Draw something in Paint and exit activity.
• In the Journal, show secondary menu on the paint object, and select: Copy to -> Clipboard
• Go to Write and try paste it.

Write does not paste, and show the following error in the log:

Traceback (most recent call last):
  File "/home/olpc/Activities/Write.activity/toolbar.py", line 158, in __paste_button_cb
    selection = clipboard.wait_for_contents('text/uri-list')
  File "/usr/lib/python2.7/site-packages/gi/types.py", line 47, in function
    return info.invoke(*args, **kwargs)
TypeError: argument target: Expected Gdk.Atom, but got str

[godiard]

Pushed fix as cfc1d3bd76ed8b6dc73402748ccaeabe58241f0c