update dependency versions

Author: Dan Richelson

requirements.txt

@@ -1,6 +1,7 @@
 backoff>=1.3.1
-CacheControl>=0.10.2
-requests>=2.10.0
-future>=0.15.2
+CacheControl>=0.11.7
+requests>=2.11.1
+future>=0.16.0
+six>=1.10.0
 strict-rfc3339>=0.7
 jsonpickle==0.9.3