build_table_schema has hardcoded Pandas version (0.20.0)
#### Code Sample, a copy-pastable example if possible

```python
import pandas
from pandas.io.json import build_table_schema
df = pandas.DataFrame(data={'col1': [1, 2], 'col2': [3, 4]})
build_table_schema(df)
```

produces 

```
{'fields': [{'name': 'index', 'type': 'integer'},
  {'name': 'col1', 'type': 'integer'},
  {'name': 'col2', 'type': 'integer'}],
 'primaryKey': ['index'],
 'pandas_version': '0.20.0'}
```
#### Problem description

The version `0.20.0` is hard coded in the function. 

#### Expected Output

`pandas.__version__` returns `'0.25.0+349.g9dc6de3.dirty'`, which should also be returned by `build_table_schema`

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

<details>

INSTALLED VERSIONS
------------------
commit           : ac6bbaf0620a634b430f444371facc165585976f
python           : 3.6.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-55-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_CA.UTF-8
LOCALE           : en_CA.UTF-8

pandas           : 0.25.0+350.gac6bbaf
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : 3.5.1
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None


</details>
