TST: failing tests at pandas-wheels
A bunch of tests are failing in the wheel-building repo (https://github.com/MacPython/pandas-wheels): https://travis-ci.org/MacPython/pandas-wheels/builds/158414872

It are mainly issues with 32 bit linux (see eg https://travis-ci.org/MacPython/pandas-wheels/jobs/158414875), but also some tests are failing for 64bit:

On 2.7:

```
======================================================================
FAIL: test_abs (pandas.tests.test_panel4d.TestPanel4d)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/venv/local/lib/python2.7/site-packages/pandas/tests/test_panel4d.py", line 303, in test_abs
    assert_frame_equal(result, expected)
  File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
    self.gen.next()
  File "/venv/local/lib/python2.7/site-packages/pandas/util/testing.py", line 2538, in assert_produces_warning
    % extra_warnings)
AssertionError: Caused unexpected warning(s): ['RuntimeWarning', 'RuntimeWarning', 'RuntimeWarning'].
```

On 3.5:

```
======================================================================
FAIL: test_pi_ops_errors (pandas.tseries.tests.test_period.TestPeriodIndexSeriesMethods)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/venv/lib/python3.5/site-packages/pandas/tseries/tests/test_period.py", line 4184, in test_pi_ops_errors
    np.add(ng, obj)
  File "/venv/lib/python3.5/site-packages/pandas/util/testing.py", line 2450, in __exit__
    raise AssertionError("{0} not raised.".format(name))
AssertionError: TypeError not raised.
======================================================================

FAIL: test_pi_sub_period (pandas.tseries.tests.test_period.TestPeriodIndexSeriesMethods)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/venv/lib/python3.5/site-packages/pandas/tseries/tests/test_period.py", line 4299, in test_pi_sub_period
    tm.assert_index_equal(result, exp)
  File "/venv/lib/python3.5/site-packages/pandas/util/testing.py", line 756, in assert_index_equal
    assertIsInstance(left, Index, '[index] ')
  File "/venv/lib/python3.5/site-packages/pandas/util/testing.py", line 947, in assertIsInstance
    raise AssertionError(err_msg.format(msg, cls, type(obj)))
AssertionError: [index] Expected type <class 'pandas.indexes.base.Index'>, found <class 'NotImplementedType'> instead
```

I can't directly reproduce, but that last one is possibly an issue with the numpy version
