Only round end_time of removal segments

We don't actually care if we chop out a segment after a keyframe,
just that the next segment should start with one. So only the
end_time requires rounding, not the start_time.

test/test_postprocessors.py

@@ -567,7 +567,7 @@ class TestModifyChaptersPP(unittest.TestCase):
         ])
         self.assertEqual(chapters, [
             {'start_time': 0, 'end_time': 2},
-            {'start_time': 3, 'end_time': 5, 'remove': True},
+            {'start_time': 2, 'end_time': 5, 'remove': True},
             {'start_time': 6, 'end_time': 10, 'remove': False},
         ])