Document the requirement for PyTest 2.8.6 or above.

Fixes #43.

pytest_doctest_test.py

@@ -37,13 +37,12 @@ def make_file(name, content=''):
 passes = make_file_factory('passes', fake=False, result=True)
 passes_too = make_file_factory('passes_too', fake=True, result=True)
 
-# Actually, it crashes if we try to remove the file :/
 passes_too.__doc__ = passes_too.__doc__.replace('>>> os.remove(name)',
                                                 '>>> pass')
 
 fails = make_file_factory('fails', fake=False, result=False)
 
-# Pytest raises an internal error when running these doctests:
+# Pytest versions below 2.8.6 raise an internal error when running these doctests:
 crashes = make_file_factory('crashes', fake=True, result=False)
 crashes_too = make_file_factory(') SyntaxError', fake=True, result=False)