Error when calculating variance of series of ndarrays
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd
# np.__version__ is 1.14.5
# pd.__version__ is 0.24.1

# create list of dicts with ndarrays
ll = []
for ii in range(2):
    dd = {0: np.ones(2)}
    ll += [dd]

# create data frame
df = pd.DataFrame(ll)

# print(df) looks as expected
#             0
# 0  [1.0, 1.0]
# 1  [1.0, 1.0]

m = df[0].mean() # works as expected
# equivalent to df[0].values.mean(axis=0)
print(m) # array([1., 1.])

v = df[0].values.var(axis=0) # yields expected result array([0., 0.])

v = df[0].var() # raises TypeError: setting an array element with a sequence.
```
#### Problem description

A Pandas series od dtype object can contain numpy.ndarrays. This ist useful to store high-dimensional data in DataFrames.
Calculating the mean of such a series works as expected. Calculating the variance however, yields an error. The calculation is easily performed by inserting .values between the series and the var call, so it is no fundamental problem.

This is the error Traceback of df[0].var()

```
Traceback (most recent call last):

  File "<ipython-input-23-be47a51ab53b>", line 1, in <module>
    df[0].var()

  File "C:\Users\Maksim\WinPython\python-3.6.3.amd64\lib\site-packages\pandas\core\generic.py", line 10976, in stat_func
    skipna=skipna, ddof=ddof)

  File "C:\Users\Maksim\WinPython\python-3.6.3.amd64\lib\site-packages\pandas\core\series.py", line 3626, in _reduce
    return op(delegate, skipna=skipna, **kwds)

  File "C:\Users\Maksim\WinPython\python-3.6.3.amd64\lib\site-packages\pandas\core\nanops.py", line 76, in _f
    return f(*args, **kwargs)

  File "C:\Users\Maksim\WinPython\python-3.6.3.amd64\lib\site-packages\pandas\core\nanops.py", line 138, in f
    raise TypeError(e)

TypeError: setting an array element with a sequence.
```

#### Expected Output

I expect 
df[0].var()
to yield the same as
df[0].values.var(axis=0)

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.24.1
pytest: 3.2.3
pip: 18.1
setuptools: 39.2.0
Cython: 0.27.2
numpy: 1.14.5
scipy: 1.1.0
pyarrow: 0.7.1
xarray: 0.9.6
IPython: 6.2.1
sphinx: 1.6.5
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: 1.5.1
bottleneck: 1.2.1
tables: None
numexpr: None
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.2
lxml.etree: 4.1.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.14
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
