Allow newer versions of python-memcached in tests

Version 1.51 is older, and newer versions have fixes for python3.

Author: SpamapS

setup.py

@@ -38,7 +38,7 @@ def run_tests(self):
     'mongodict',
     'pyasn1',
     'pymongo==3.0.1',
-    'python-memcached == 1.51',
+    'python-memcached >= 1.51',
     'pytest',
     'mako',
     #'pytest-coverage',