CI: Build failures
e.g. https://travis-ci.org/pandas-dev/pandas/jobs/326347971

```
[script_single]
pytest -s -m single --strict --cov=pandas --cov-report xml:/tmp/cov-single.xml --junitxml=/tmp/single.xml --skip-slow --skip-network pandas
Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/config.py", line 328, in _getconftestmodules
    return self._path2confmods[path]
KeyError: local('/home/travis/build/pandas-dev/pandas/pandas')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/config.py", line 359, in _importconftest
    return self._conftestpath2mod[conftestpath]
KeyError: local('/home/travis/build/pandas-dev/pandas/pandas/conftest.py')
During handling of the above exception, another exception occurred:
RuntimeError: module compiled against API version 0xc but this version of numpy is 0xb
Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/config.py", line 328, in _getconftestmodules
    return self._path2confmods[path]
KeyError: local('/home/travis/build/pandas-dev/pandas/pandas')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/config.py", line 359, in _importconftest
    return self._conftestpath2mod[conftestpath]
KeyError: local('/home/travis/build/pandas-dev/pandas/pandas/conftest.py')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/build/pandas-dev/pandas/pandas/__init__.py", line 26, in <module>
    from pandas._libs import (hashtable as _hashtable,
  File "/home/travis/build/pandas-dev/pandas/pandas/_libs/__init__.py", line 4, in <module>
    from .tslib import iNaT, NaT, Timestamp, Timedelta, OutOfBoundsDatetime
  File "pandas/_libs/tslibs/conversion.pxd", line 11, in init pandas._libs.tslib
    cdef class _TSObject:
  File "pandas/_libs/tslibs/conversion.pyx", line 1, in init pandas._libs.tslibs.conversion
    # -*- coding: utf-8 -*-
ImportError: numpy.core.multiarray failed to import
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/config.py", line 365, in _importconftest
    mod = conftestpath.pyimport()
  File "/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/py/_path/local.py", line 668, in pyimport
    __import__(modname)
  File "/home/travis/build/pandas-dev/pandas/pandas/__init__.py", line 35, in <module>
    "the C extensions first.".format(module))
ImportError: C extension: numpy.core.multiarray failed to import not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.
ERROR: could not load /home/travis/build/pandas-dev/pandas/pandas/conftest.py

```

Looking into it now.