Hitting AmbiguousTimeError when exiting DST using .loc
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import arrow
import datetime
from dateutil import tz

test_time = arrow.get(datetime.datetime(2015,11,1), "US/Pacific").datetime
timestamps = [test_time + datetime.timedelta(seconds=i * 900) for i in range(16)]
my_series = pd.Series([0], index=[timestamps[0].replace(tzinfo=None)])
my_series = my_series.tz_localize(tz.gettz("US/Pacific"), ambiguous='infer')

for ts in timestamps:
    my_series.loc[ts] = 1
```
#### Problem description

I have a program that is running as a service and slowly adds timestamps to a pandas series. When running simulations, it always breaks when exiting DST.

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 38.2.3
Cython: 0.24
numpy: 1.13.3
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 1.0b10
sqlalchemy: 1.2.4
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
