require pytest>=2.5 when running tests from setup.py

Author: andymccurdy

setup.py

@@ -42,7 +42,7 @@ def run_tests(self):
     keywords=['Redis', 'key-value store'],
     license='MIT',
     packages=['redis'],
-    tests_require=['pytest'],
+    tests_require=['pytest>=2.5.0'],
     cmdclass={'test': PyTest},
     classifiers=[
         'Development Status :: 5 - Production/Stable',