BUG: JSON iso date formatting incorrect on linux 32 bit

``` python
In [2]: pd.json.dumps(pd.Timestamp('2013-01-01'), iso_dates=True)
Out[2]: '"699470488266606557-01-01T00:00:00.000Z"'
```

should be

``` python
In [4]: pd.json.dumps(pd.Timestamp('2013-01-01'), iso_dates=True)
Out[4]: '"2013-01-01T00:00:00.000Z"'
```
