fixes division by zero error for kurt()
Currently if `kurt()` is called on a `Series` with equal values, it throws a `ZeroDivisionError`

```
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: s = pd.Series(np.ones(5))
In [4]: s.kurt()
ZeroDivisionError: float division by zero
```

This is not consistent with the case when `kurt()` is called on a `DataFrame` of equal values

```
In [5]: df = pd.DataFrame(np.ones((5, 5)))
In [6]: df.kurt()
Out[6]:
0    0
1    0
2    0
3    0
4    0
dtype: float64
```

with this patch `s.kurt()` will return `0` instead of throwing.
