TypeError: rank() got an unexpected keyword argument 'numeric_only'
```
In [19]: df = DataFrame({'a':['A1', 'A1', 'A1'], 'b':['B1','B1','B2'], 'c':1})

In [20]: df.set_index('a').groupby('b').rank(method='first')
Out[20]: 
    c
a    
A1  1
A1  2
A1  1

In [21]: df.set_index('a').groupby('c').rank(method='first')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-21-6b8d4cae9d91> in <module>()
----> 1 df.set_index('a').groupby('c').rank(method='first')

/home/nicolas/Git/pandas/pandas/core/groupby.pyc in rank(self, axis, numeric_only, method, na_option, ascending, pct)

/home/nicolas/Git/pandas/pandas/core/groupby.pyc in wrapper(*args, **kwargs)
    618                     # mark this column as an error
    619                     try:
--> 620                         return self._aggregate_item_by_item(name, *args, **kwargs)
    621                     except (AttributeError):
    622                         raise ValueError

/home/nicolas/Git/pandas/pandas/core/groupby.pyc in _aggregate_item_by_item(self, func, *args, **kwargs)
   3076             # GH6337
   3077             if not len(result_columns) and errors is not None:
-> 3078                 raise errors
   3079 
   3080         return DataFrame(result, columns=result_columns)

TypeError: rank() got an unexpected keyword argument 'numeric_only'
```

I'm trying to obtain what I would get with a `row_number()` in SQL...

Notice that if I replace the value in the `'c'` column with the string `'1'`, then even `df.set_index('a').groupby('b').rank(method='first')` fails.

Am I doing something wrong?
