BUG: index union between Int64 and non orderable raise AttributeError
``` python
x = pd.Index([1, 2, 3])
y = pd.Index(['a', 'b', 'c'])
x.union(y)
```

Sorry no stack trace (on a remote server). Raises with an AttributeError and a message about no attribute is_monotonic_increasing. This is on 0.15.1

I think I saw a thing in the release notes about adding that property in 0.15.1 and I can confirm that the example worked in 0.15.0.
