Skip memcache test if necessary

Change-Id: Ib670fa8588a5cbbe1412e10dface98bee7d5dbeb

Author: Jon Wayne Parrott

appengine/flexible/memcache/main_test.py

@@ -14,9 +14,15 @@
 
 import pytest
 
-import main
+try:
+    import main
+except ImportError:
+    main = None
 
 
+@pytest.mark.skipif(
+    not main,
+    reason='pylibmc not installed.')
 def test_index():
     try:
         main.memcache_client.set('counter', 0)