Switch test to 2017.7

Change-Id: I99298f2a6b5d794a0ece06746f92eb5d3fd620ba
diff --git a/.travis.yml b/.travis.yml
index 7ea81dc..b367f1f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,7 +17,7 @@
   - bundle install
 
 env:
-    - PLATFORM=trevorj/salty-whales:xenial SUITE=default
+    - PLATFORM=trevorj/salty-whales:xenial-2017.7 SUITE=default
 
 before_script:
   - set -o pipefail