misrepresented fractional seconds in timestamps and timedeltas
#### Code Sample

```python
>>> timestamp = 1490193630.8
>>> pd.to_timedelta(timestamp, unit='s')
Timedelta('17247 days 14:40:30.799999')
```

However, this works:
```python
>>> pd.to_timedelta(timestamp*1e9, unit='ns')
Timedelta('17247 days 14:40:30.800000')
```
#### Problem description

Depending upon the value of the float to be converted to a timestamp or timedelta and on the unit, the resulting timestamp and timedelta will occasionally misrepresent the fractional part of the input float.

#### Expected Output
`Timedelta('17247 days 14:40:30.8')`
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: a4482db46535b018053b4351689ccc3c79257d8c
python: 3.6.5.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: 0.24.0.dev0+716.ga4482db46
pytest: 3.5.1
pip: 9.0.3
setuptools: 39.0.1
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: 0.9.0.post1
xarray: 0.10.3
IPython: 6.4.0
sphinx: 1.7.4
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: 0.8.1
psycopg2: None
jinja2: 2.10
s3fs: 0.1.5
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
