Unexpected results in Numpy datetime64 comparison within DataFrame 
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np
from datetime import datetime,timedelta

threshold = np.datetime64(datetime.today()+timedelta(weeks=3))
df[threshold < df['date']]
df[df['date'] < threshold]

```
#### Problem description

As the two comparisons above show, they should present opposite results. Instead, both of them return the same result, as if df['date'] was always the first comparison operand.

#### Expected Output

The picture below illustrates the issue. It was expected that the line ``df[threshold < df['date']]`` would result in an empty DataFrame. 
![screenshot from 2017-07-05 15-17-58](https://user-images.githubusercontent.com/5377246/27878548-fe104da6-6194-11e7-8f53-a6158d8851df.png)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-83-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 34.4.1
Cython: None
numpy: 1.12.1
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.3
html5lib: None
httplib2: 0.10.3
apiclient: 1.6.2
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: 2.38.0
pandas_datareader: None


</details>
