tests: fix test_filter_prev_values expected result

pandas 0.17.1 has a more accurate assert_series_equal() that makes
test_filter_prev_values fail because the index name is different:

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FAIL: Trigger works with a filter that depends on previous values of the same pivot
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Traceback (most recent call last):
  File "/home/javi/src/trappy/tests/test_stats.py", line 227, in test_filter_prev_values
    assert_series_equal(expected, trigger.generate("blank"))
  File "/usr/lib/python2.7/dist-packages/pandas/util/testing.py", line 924, in assert_series_equal
    obj='{0}.index'.format(obj))
  File "/usr/lib/python2.7/dist-packages/pandas/util/testing.py", line 694, in assert_index_equal
    assert_attr_equal('names', left, right, obj=obj)
  File "/usr/lib/python2.7/dist-packages/pandas/util/testing.py", line 729, in assert_attr_equal
    left_attr, right_attr)
  File "/usr/lib/python2.7/dist-packages/pandas/util/testing.py", line 819, in raise_assert_detail
    raise AssertionError(msg)
AssertionError: Series.index are different

Attribute "names" are different
[left]:  [u'Time']
[right]: [None]

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The series that trigger generates don't have a name for the index.
Remove the named index from the expected output so that the test passes
again.

0 files changed, 0 insertions, 0 deletions