BLD: wheel building is failing
after the latest round of merges, seems wheel buidling is failing:

https://travis-ci.org/MacPython/pandas-wheels/builds/548526771

```
=================================== FAILURES ===================================
_______________________ test_missing_required_dependency _______________________
[gw1] linux -- Python 3.5.5 /venv/bin/python
    def test_missing_required_dependency():
        # GH 23868
        # use the -S flag to disable site-packages
        call = ['python', '-S', '-c', 'import pandas']
    
        with pytest.raises(subprocess.CalledProcessError) as exc:
            subprocess.check_output(call, stderr=subprocess.STDOUT)
    
        output = exc.value.stdout.decode()
>       assert all(x in output for x in ['numpy', 'pytz', 'dateutil'])
E       assert False
E        +  where False = all(<generator object test_missing_required_dependency.<locals>.<genexpr> at 0x7f6e59bd4c50>)
/venv/lib/python3.5/site-packages/pandas/tests/test_downstream.py:145: AssertionError
=============================== warnings summary ===============================
```