Fix to_list work properly in pandas < 0.24

[itholic]

Koalas doesn't support to_list in pandas<0.24 because pandas<0.24 doesn't support to_list as an alias for tolist.

>>> pd.__version__
'0.23.2'
>>> pser.to_list()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/haejoon.lee/opt/miniconda3/envs/koalas-dev-3.6/lib/python3.6/site-packages/pandas/core/generic.py", line 4372, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'Series' object has no attribute 'to_list'
>>> pser.tolist()
[1, 2, 3]

This PR fix it to make working to_list properly in pandas<0.24.

[itholic]

I'm pretty sure this is minor fix. Please feel free to comment anytime !