infer_freq broken in 0.13.1
``` python
In [3]: dates = pd.date_range('01-Jan-2015', '01-Jan-2016', freq='MS')
   ...: s = pd.TimeSeries(1, dates)
   ...: pd.infer_freq(s)
   ...: 
Traceback (most recent call last):

  File "<ipython-input-3-a7de7a7e9245>", line 3, in <module>
    pd.infer_freq(s)

  File "C:\dev\bin\Anaconda\lib\site-packages\pandas\tseries\frequencies.py", line 656, in infer_freq
    inferer = _FrequencyInferer(index, warn=warn)

  File "C:\dev\bin\Anaconda\lib\site-packages\pandas\tseries\frequencies.py", line 680, in __init__
    self.is_monotonic = self.index.is_monotonic

  File "C:\dev\bin\Anaconda\lib\site-packages\pandas\core\generic.py", line 1815, in __getattr__
    (type(self).__name__, name))

AttributeError: 'Series' object has no attribute 'is_monotonic'


In [4]: pd.__version__
Out[4]: '0.13.1'
```
