travis: bump minor versions of Cassandra

.travis.yml

@@ -13,10 +13,10 @@ env:
   global:
     - GOMAXPROCS=2
   matrix:
-    - CASS=2.0.16 AUTH=false
-    - CASS=2.1.5  AUTH=false
-    - CASS=2.1.5  AUTH=true
-    - CASS=2.2.0  AUTH=false
+    - CASS=2.0.17 AUTH=false
+    - CASS=2.1.9  AUTH=false
+    - CASS=2.1.9  AUTH=true
+    - CASS=2.2.1  AUTH=false
 
 go:
   - 1.4