Regression in 0.20: IO functions can't handle pathlib.Path objects anymore
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd
from pathlib import Path
d = pd.DataFrame(np.random.randn(25).reshape((5, 5)))
d.to_pickle('test.pickle')
d2 = pd.read_pickle(Path('test.pickle'))
```
#### Problem description

In 0.19, Pandas' IO functions accepted PathLike objects as per PEP 519. This is broken in 0.20.1.

#### Expected Output

*nothing, code should run without errors or output*

#### Actual Output

```
Traceback (most recent call last):
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 83, in try_read
    return read_wrapper(lambda f: pkl.load(f))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 83, in <lambda>
    return read_wrapper(lambda f: pkl.load(f))
TypeError: file must have 'read' and 'readline' attributes

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 88, in try_read
    lambda f: pc.load(f, encoding=encoding, compat=False))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 88, in <lambda>
    lambda f: pc.load(f, encoding=encoding, compat=False))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/compat/pickle_compat.py", line 187, in load
    fh.seek(0)
AttributeError: 'PosixPath' object has no attribute 'seek'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 94, in read_pickle
    return try_read(path)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 92, in try_read
    lambda f: pc.load(f, encoding=encoding, compat=True))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 92, in <lambda>
    lambda f: pc.load(f, encoding=encoding, compat=True))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/compat/pickle_compat.py", line 187, in load
    fh.seek(0)
AttributeError: 'PosixPath' object has no attribute 'seek'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 83, in try_read
    return read_wrapper(lambda f: pkl.load(f))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 83, in <lambda>
    return read_wrapper(lambda f: pkl.load(f))
TypeError: file must have 'read' and 'readline' attributes

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 88, in try_read
    lambda f: pc.load(f, encoding=encoding, compat=False))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 88, in <lambda>
    lambda f: pc.load(f, encoding=encoding, compat=False))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/compat/pickle_compat.py", line 187, in load
    fh.seek(0)
AttributeError: 'PosixPath' object has no attribute 'seek'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 97, in read_pickle
    return try_read(path, encoding='latin1')
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 92, in try_read
    lambda f: pc.load(f, encoding=encoding, compat=True))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 68, in read_wrapper
    return func(f)
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/io/pickle.py", line 92, in <lambda>
    lambda f: pc.load(f, encoding=encoding, compat=True))
  File "/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/pandas/compat/pickle_compat.py", line 187, in load
    fh.seek(0)
AttributeError: 'PosixPath' object has no attribute 'seek'
```

#### Output of ``pd.show_versions()``

<details>
>>> pd.show_versions()
/home/mruffalo/.opt/python3.6-venv/lib/python3.6/site-packages/xarray/core/formatting.py:16: FutureWarning: The pandas.tslib module is deprecated and will be removed in a future version.
  from pandas.tslib import OutOfBoundsDatetime

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-51-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: 0.9.1
IPython: 6.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>