compare a column of np.timedelta64 with a single np.timedelta64 throws ValueError
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

import pandas as pd
import numpy as np

# create a dateframe with a column "duration", which contains timedelta of 5 minutes
p=pd.DataFrame({"duration":[np.timedelta64(5, "m")]})

# compare this column with timedelta of 4 minutes
p.duration<np.timedelta64(4, "m")

#this should result in a series of one bool which is false.
```
#### Problem description

When you try to compare a column of np.timedelta64 with a single np.timedelta64, version 0.22.0 throws 

**ValueError: cannot set a Timedelta with a non-timedelta**

This worked in previous version 0.20.3 and returned the correct series of bools

#### Expected Output

This is the output as of version 0.20.3

0    False
Name: duration, dtype: bool

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

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.22.0
pytest: 2.7.1
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.22.1
numpy: 1.11.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 4.0.0
sphinx: 1.3.1
patsy: 0.4.1
dateutil: 2.4.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 1.5.1
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 1.0.0
xlsxwriter: 0.7.3
lxml: 3.6.0
bs4: 4.6.0
html5lib: None
sqlalchemy: 1.0.12
pymysql: 0.6.7.None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
