Fix hard-coded pandas version to infer the current
Fix hard-coded pandas version to infer the current version in `pd.io.json.build_table_schema` output. 

```python
>>> import pandas as pd
>>> pd.__version__
'0.23.4'
>>> df = pd.DataFrame({'A': [1,2], 'B': ['a','b']})
>>> # BEFORE
>>> pd.io.json.build_table_schema(df, index=0)['pandas_version']
'0.20.0'
>>> # AFTER
>>> pd.io.json.build_table_schema(df, index=0)['pandas_version']
'0.23.4'
```
