TST: open image file in binary mode

Fix test error on Windows:
```
======================================================================
ERROR: test_io.test_imread
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\nose\case.py", line 198, in runTest
    self.test(*self.arg)
  File "X:\Python34\lib\site-packages\numpy\testing\decorators.py", line 146, in skipper_func
    return f(*args, **kwargs)
  File "X:\Python34\lib\site-packages\scipy\ndimage\tests\test_io.py", line 26, in test_imread
    img = ndi.imread(fobj, mode="RGB")
  File "X:\Python34\lib\site-packages\scipy\ndimage\io.py", line 43, in imread
    im = Image.open(fname)
  File "X:\Python34\lib\site-packages\PIL\Image.py", line 1997, in open
    prefix = fp.read(16)
  File "X:\Python34\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x9d in position 84: character maps to <undefined>

```