Test errors for numpy 1.11 / pandas 0.18.0
See: https://travis-ci.org/matthew-brett/manylinux-testing/jobs/119188668#L3214

```
======================================================================
ERROR: test_range_slice_day (pandas.tseries.tests.test_period.TestPeriodIndex)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/matthew-brett/manylinux-testing/venv/lib/python2.7/site-packages/pandas/tseries/tests/test_period.py", line 2603, in test_range_slice_day
    idx[v:]
  File "/home/travis/build/matthew-brett/manylinux-testing/venv/lib/python2.7/site-packages/pandas/tseries/base.py", line 175, in __getitem__
    result = getitem(key)
IndexError: failed to coerce slice entry of type str to integer
======================================================================
ERROR: test_range_slice_seconds (pandas.tseries.tests.test_period.TestPeriodIndex)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/matthew-brett/manylinux-testing/venv/lib/python2.7/site-packages/pandas/tseries/tests/test_period.py", line 2660, in test_range_slice_seconds
    idx[v:]
  File "/home/travis/build/matthew-brett/manylinux-testing/venv/lib/python2.7/site-packages/pandas/tseries/base.py", line 175, in __getitem__
    result = getitem(key)
IndexError: failed to coerce slice entry of type str to integer
```

No such error testing against numpy 1.10.4 : https://travis-ci.org/matthew-brett/manylinux-testing/jobs/119039450#L523
