BUG: repr of series fails repr in the presence of a Float64Index that is not monotonic (GH5557)

closes #5557

```
In [4]: s.value_counts()
Out[4]: 
0        85
70        3
78        3
2584      1
3001      1
734       1
163       1
3374      1
9643      1
42        1
8172      1
931       1
802       1
67744     1
2716      1
...
775      1
36605    1
1630     1
605      1
347      1
729      1
88       1
854      1
34773    1
2769     1
3536     1
67404    1
264      1
1228     1
324      1
Length: 61, dtype: int64
```