TypeError bug when using pandas.to_timedelta (astype() unexpected argument 'copy')
#### Code Sample

Running the following code:

``` python
import numpy as np
import pandas as pd
df = pd.DataFrame(np.random.randn(6,4),columns=list('ABCD'))
pd.to_timedelta(df.index,unit='s')
```

returns a `TypeError`:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-cf2e27d56944> in <module>()
      1 df = pd.DataFrame(np.random.randn(6,4),columns=list('ABCD'))
----> 2 pd.to_timedelta(df.index,unit='s')

~/anaconda/lib/python3.5/site-packages/pandas/util/decorators.py in wrapper(*args, **kwargs)
     89                 else:
     90                     kwargs[new_arg_name] = new_arg_value
---> 91             return func(*args, **kwargs)
     92         return wrapper
     93     return _deprecate_kwarg

~/anaconda/lib/python3.5/site-packages/pandas/tseries/timedeltas.py in to_timedelta(arg, unit, box, errors, coerce)
     91         return Series(values, index=arg.index, name=arg.name, dtype='m8[ns]')
     92     elif isinstance(arg, ABCIndexClass):
---> 93         return _convert_listlike(arg, box=box, unit=unit, name=arg.name)
     94     elif is_list_like(arg) and getattr(arg, 'ndim', 1) == 1:
     95         return _convert_listlike(arg, box=box, unit=unit)

~/anaconda/lib/python3.5/site-packages/pandas/tseries/timedeltas.py in _convert_listlike(arg, box, unit, name)
     73         elif is_integer_dtype(arg):
     74             value = arg.astype('timedelta64[{0}]'.format(
---> 75                 unit)).astype('timedelta64[ns]', copy=False)
     76         else:
     77             value = tslib.array_to_timedelta64(

TypeError: astype() got an unexpected keyword argument 'copy'
```
#### Expected Output

Should instead return the following:

```
TimedeltaIndex(['00:00:00', '00:00:01', '00:00:02', '00:00:03', '00:00:04',
                '00:00:05'],
               dtype='timedelta64[ns]', freq=None)
```

which is what I get if I manually (for testing) remove the copy argument in the `timedeltas.py`
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 26.1.1
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.3.1
numexpr: 2.6.1
matplotlib: 1.5.2
openpyxl: 2.3.5
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None
```
