Temporary fix ExpectedFailure does not handle SystemExit on python 2.

Actually the test not only is expected to fail but it misses a file
which causes an error instead of a failure on python 2. Should be
addressed by #25.

tests/test_usecase.py

@@ -212,7 +212,7 @@ class TestAdd(DataCommandTestCase):
         with self.assertRaises(SystemExit):
             self.execute_cmds(cmds)
 
-    @unittest.expectedFailure
+    @unittest.skip
     def test_leading_citekey_space(self):
         cmds = ['pubs init',
                 'pubs add /data/leadingspace.bib',
@@ -221,8 +221,6 @@ class TestAdd(DataCommandTestCase):
         self.execute_cmds(cmds)
 
 
-
-
 class TestList(DataCommandTestCase):
 
     def test_list(self):