BUG: 32/64-bit issue with indexing

https://groups.google.com/forum/#!topic/pydata/90QNz8EY74Q

fails on 32-bit

```
In [1]: df = pd.DataFrame({"A": range(2), "B": [pd.Timestamp('2000-01-1')]*2})

In [2]: df
Out[2]: 
   A          B
0  0 2000-01-01
1  1 2000-01-01

[2 rows x 2 columns]

In [3]: df.groupby("A")["B"].transform(min)
Out[3]: 
0   2000-01-01
1   2000-01-01
Name: B, dtype: datetime64[ns]
```
