numpy binary incompatibility for pandas 0.23.4 and numpy 1.15 with pip and .whl
#### Code Sample, a copy-pastable example if possible

A simple pandas import

```python
import pandas as pd
```

#### Problem description

Several `RuntimeWarnings` in Python 2 or Python 3. I think it's due to a different version of numpy between:

* the version of numpy used to compile pandas and built the `.whl` file available on Pypi https://pypi.org/project/pandas/#files
* the latest version of numpy on Pypi, i.e. 1.15

I think the only version constraint is `numpy >= 1.9`. When you install with pip, it gets the latest version of numpy.

I wonder how I can retrieve the version of numpy used to compile pandas and build the `.whl` file. If I reinstall `numpy==1.14.5`, I don't have the `RuntimeWarning`.

#### Outputs

```
venv/local/lib/python2.7/site-packages/pandas/_libs/__init__.py:4: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
  from .tslib import iNaT, NaT, Timestamp, Timedelta, OutOfBoundsDatetime
venv/local/lib/python2.7/site-packages/pandas/__init__.py:26: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
  from pandas._libs import (hashtable as _hashtable,
venv/local/lib/python2.7/site-packages/pandas/core/dtypes/common.py:6: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
  from pandas._libs import algos, lib
venv/local/lib/python2.7/site-packages/pandas/core/util/hashing.py:7: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
  from pandas._libs import hashing, tslib
venv/local/lib/python2.7/site-packages/pandas/core/indexes/base.py:7: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
...
and other `_libs` imports
```

#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.15.final.0
python-bits: 64
OS: Linux
OS-release: 4.16.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.4
pip: 18.0
setuptools: 40.2.0
numpy: 1.15.0
IPython: 5.8.0
dateutil: 2.7.3
pytz: 2018.5
```


</details>
