Add explicit dependency on umemcache.

I don't really know why this is necessary but I'm hoping it will
get the tests passing on CI. It shouldn't be required for actual
running of the server.

requirements.txt

@@ -10,5 +10,6 @@ configparser==3.5
 mozsvc==0.9
 futures==3.0
 soupsieve==1.9.5
+umemcache==1.6.3
 https://github.com/mozilla-services/tokenserver/archive/1.5.11.zip
 https://github.com/mozilla-services/server-syncstorage/archive/1.8.0.zip