Slicing on series with integer index behaves positional.
``` python
In [12]: s = pandas.Series(range(5), index=[0, 6, 20, 100, 134])

In [13]: s
Out[13]:
0      0
6      1
20     2
100    3
134    4

In [14]: s[1:3]
Out[14]:
6     1
20    2
```

Whereas `s.ix[]` works label based.

``` python
In [15]: s.ix[1:3]
Out[15]: Series([], dtype=int64)
```
