Rounding valid timestamps near daylight savings jumps should not throw NonExistentTimeError
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

# valid time rounds incorrectly
pd.Timestamp('2018-03-11 01:59:00-0600', tz='America/Chicago').round(freq='5min')
# throws NonExistentTimeError: 2018-03-11 02:00:00
# should return pd.Timestamp('2018-03-11 03:00:00-0500', tz='America/Chicago')
```
#### Problem description

The timestamp is a valid time, so it should round to another valid timestamp. Specifically, it should round to the nearest 5min frequency valid time, which is the moment after the clock jumps.

#### Expected Output
```pd.Timestamp('2018-03-11 03:00:00-0500', tz='America/Chicago')```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-1070-aws
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: None
pip: 18.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.0
scipy: 1.1.0
pyarrow: 0.9.0
xarray: None
IPython: 6.5.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.6
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: 0.4.0
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.10
pymysql: None
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
