Erratic behavior using query() and timedelta
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
d = pd.DataFrame([pd.Timedelta(5,'D')], columns=['dt'])
d.query('dt =="5 days"', engine='numexpr')
# works: 
#      dt
# 0 5 days

d.query('dt =="4 days"', engine='numexpr')
# works:
#Empty DataFrame
#Columns: [dt]
#Index: []

d.query('dt <"5 days"', engine='numexpr')
# Does not work
# ValueError: unknown type timedelta64[ns]


```
#### Problem description

Using query on timedelta values has erratic behavior, and was working before. I can't see how suddenly, less than, and greater than operations using `query` do not work, but == does.

#### Expected Output
```python
import pandas as pd
d = pd.DataFrame([pd.Timedelta(5,'D')], columns=['dt'])


d.query('dt <"5 days"')
#EXPECTED:
#      dt
# 0 5 days`
```
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: None
pip: 18.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.8
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.9
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.7.0

</details>
