TST: Fix test_backend_ps failures on Windows

```
======================================================================
FAIL: matplotlib.tests.test_backend_ps.test_savefig_to_stringio
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python27-x64\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "X:\Python27-x64\lib\site-packages\matplotlib\testing\decorators.py", line 110, in wrapped_function
    func(*args, **kwargs)
  File "X:\Python27-x64\lib\site-packages\matplotlib\tests\test_backend_ps.py", line 47, in test_savefig_to_stringio
    _test_savefig_to_stringio()
  File "X:\Python27-x64\lib\site-packages\matplotlib\tests\test_backend_ps.py", line 42, in _test_savefig_to_stringio
    assert values[1] == values[2]
AssertionError

======================================================================
FAIL: matplotlib.tests.test_backend_ps.test_savefig_to_stringio_eps
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python27-x64\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "X:\Python27-x64\lib\site-packages\matplotlib\testing\decorators.py", line 110, in wrapped_function
    func(*args, **kwargs)
  File "X:\Python27-x64\lib\site-packages\matplotlib\tests\test_backend_ps.py", line 68, in test_savefig_to_stringio_eps
    _test_savefig_to_stringio(format='eps')
  File "X:\Python27-x64\lib\site-packages\matplotlib\tests\test_backend_ps.py", line 42, in _test_savefig_to_stringio
    assert values[1] == values[2]
AssertionError
```