BLD: wheel building on 2.7 broke
https://travis-ci.org/MacPython/pandas-wheels/builds/442490830

I pinned pytest=3.8.2 and I think this resolves, but 

```
running: pytest --skip-slow --skip-network /venv/local/lib/python2.7/site-packages/pandas
============================= test session starts ==============================
platform linux2 -- Python 2.7.6, pytest-3.9.1, py-1.7.0, pluggy-0.8.0
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/io/tmp_for_test/.hypothesis/examples')
rootdir: /venv/local/lib/python2.7/site-packages/pandas, inifile:
plugins: hypothesis-3.78.0
collected 0 items / 1 errors
/venv/local/lib/python2.7/site-packages/hypothesis/_settings.py:197: HypothesisDeprecationWarning: 
The timeout setting is deprecated and will be removed in a future version of
Hypothesis. To get the future behaviour set ``timeout=hypothesis.unlimited``
instead (which will remain valid for a further deprecation period after this
setting has gone away).
  note_deprecation(d.deprecation_message, self)
==================================== ERRORS ====================================
______________________________ ERROR collecting  _______________________________
/venv/local/lib/python2.7/site-packages/pluggy/manager.py:80: in register
    % (plugin_name, plugin, self._name2plugin)
E   ValueError: Plugin already registered: /venv/local/lib/python2.7/site-packages/pandas/conftest.py=<module 'pandas.conftest' from '/venv/local/lib/python2.7/site-packages/pandas/conftest.py'>
E   {'cacheprovider': <module '_pytest.cacheprovider' from '/venv/local/lib/python2.7/site-packages/_pytest/cacheprovider.pyc'>, 'debugging': <module '_pytest.debugging' from '/venv/local/lib/python2.7/site-packages/_pytest/debugging.pyc'>, 'pytestconfig': <_pytest.config.Config object at 0x7f6a782e7dd0>, 'helpconfig': <module '_pytest.helpconfig' from '/venv/local/lib/python2.7/site-packages/_pytest/helpconfig.pyc'>, 'runner': <module '_pytest.runner' from '/venv/local/lib/python2.7/site-packages/_pytest/runner.pyc'>, 'unittest': <module '_pytest.unittest' from '/venv/local/lib/python2.7/site-packages/_pytest/unittest.pyc'>, 'pastebin': <module '_pytest.pastebin' from '/venv/local/lib/python2.7/site-packages/_pytest/pastebin.pyc'>, 'skipping': <module '_pytest.skipping' from '/venv/local/lib/python2.7/site-packages/_pytest/skipping.pyc'>, 'session': <Session 'pandas'>, 'lfplugin': <_pytest.cacheprovider.LFPlugin object at 0x7f6a77f47850>, 'fixtures': <module '_pytest.fixtures' from '/venv/local/lib/python2.7/site-packages/_pytest/fixtures.pyc'>, 'tmpdir': <module '_pytest.tmpdir' from '/venv/local/lib/python2.7/site-packages/_pytest/tmpdir.pyc'>, 'capture': <module '_pytest.capture' from '/venv/local/lib/python2.7/site-packages/_pytest/capture.pyc'>, 'terminalreporter': <_pytest.terminal.TerminalReporter object at 0x7f6a77f47310>, 'hypothesispytest': <module 'hypothesis.extra.pytestplugin' from '/venv/local/lib/python2.7/site-packages/hypothesis/extra/pytestplugin.py'>, 'assertion': <module '_pytest.assertion' from '/venv/local/lib/python2.7/site-packages/_pytest/assertion/__init__.pyc'>, 'mark': <module '_pytest.mark' from '/venv/local/lib/python2.7/site-packages/_pytest/mark/__init__.pyc'>, 'terminal': <module '_pytest.terminal' from '/venv/local/lib/python2.7/site-packages/_pytest/terminal.pyc'>, 'nfplugin': <_pytest.cacheprovider.NFPlugin object at 0x7f6a77f47890>, 'main': <module '_pytest.main' from '/venv/local/lib/python2.7/site-packages/_pytest/main.pyc'>, 'nose': <module '_pytest.nose' from '/venv/local/lib/python2.7/site-packages/_pytest/nose.pyc'>, 'warnings': <module '_pytest.warnings' from '/venv/local/lib/python2.7/site-packages/_pytest/warnings.pyc'>, 'python': <module '_pytest.python' from '/venv/local/lib/python2.7/site-packages/_pytest/python.pyc'>, 'recwarn': <module '_pytest.recwarn' from '/venv/local/lib/python2.7/site-packages/_pytest/recwarn.pyc'>, 'funcmanage': <_pytest.fixtures.FixtureManager object at 0x7f6a77f47fd0>, 'monkeypatch': <module '_pytest.monkeypatch' from '/venv/local/lib/python2.7/site-packages/_pytest/monkeypatch.pyc'>, 'resultlog': <module '_pytest.resultlog' from '/venv/local/lib/python2.7/site-packages/_pytest/resultlog.pyc'>, '/venv/local/lib/python2.7/site-packages/pandas/conftest.py': <module 'pandas.conftest' from '/venv/local/lib/python2.7/site-packages/pandas/conftest.py'>, 'logging': <module '_pytest.logging' from '/venv/local/lib/python2.7/site-packages/_pytest/logging.pyc'>, '140095260630416': <_pytest.config.PytestPluginManager object at 0x7f6a783e9d90>, 'logging-plugin': <_pytest.logging.LoggingPlugin object at 0x7f6a77f47a10>, 'setuponly': <module '_pytest.setuponly' from '/venv/local/lib/python2.7/site-packages/_pytest/setuponly.pyc'>, 'capturemanager': <_pytest.capture.CaptureManager object at 0x7f6a77fb51d0>, 'junitxml': <module '_pytest.junitxml' from '/venv/local/lib/python2.7/site-packages/_pytest/junitxml.pyc'>, 'doctest': <module '_pytest.doctest' from '/venv/local/lib/python2.7/site-packages/_pytest/doctest.pyc'>, 'setupplan': <module '_pytest.setupplan' from '/venv/local/lib/python2.7/site-packages/_pytest/setupplan.pyc'>, 'freeze_support': <module '_pytest.freeze_support' from '/venv/local/lib/python2.7/site-packages/_pytest/freeze_support.pyc'>}
=============================== warnings summary ===============================
/venv/local/lib/python2.7/site-packages/_pytest/assertion/rewrite.py:269: PytestWarning: Module already imported so cannot be rewritten: hypothesis
  self.config,
```

not sure why this doesn't break are regular builds.