[Travis] Adds pymemcache requirement to run cache backend tests

Issue #3079

docs/includes/installation.txt

@@ -75,7 +75,10 @@ Transports and Backends
     for using Amazon SQS as a message transport (*experimental*).
 
 :celery[memcache]:
-    for using memcached as a result backend.
+    for using memcached as a result backend (using pylibmc)
+
+:celery[pymemcache]:
+    for using memcached as a result backend (pure-python implementation).
 
 :celery[cassandra]:
     for using Apache Cassandra as a result backend with DataStax driver.

requirements/extras/pymemcache.txt

@@ -0,0 +1 @@
+python-memcached

requirements/test-ci-base.txt

@@ -3,4 +3,5 @@ codecov
 -r extras/redis.txt
 -r extras/mongodb.txt
 -r extras/sqlalchemy.txt
+-r extras/pymemcache.txt
 -r dev.txt

setup.py

@@ -196,9 +196,9 @@ def extras(*p):
 
 # Celery specific
 features = set([
-    'auth', 'cassandra', 'elasticsearch', 'memcache', 'couchbase', 'threads',
-    'eventlet', 'gevent', 'msgpack', 'yaml', 'redis',
-    'mongodb', 'sqs', 'couchdb', 'riak', 'beanstalk', 'zookeeper',
+    'auth', 'cassandra', 'elasticsearch', 'memcache', 'pymemcache',
+    'couchbase', 'threads', 'eventlet', 'gevent', 'msgpack', 'yaml',
+    'redis', 'mongodb', 'sqs', 'couchdb', 'riak', 'beanstalk', 'zookeeper',
     'zeromq', 'sqlalchemy', 'librabbitmq', 'pyro', 'slmq',
 ])
 extras_require = dict((x, extras(x + '.txt')) for x in features)