Switch test to 2017.7

Change-Id: I99298f2a6b5d794a0ece06746f92eb5d3fd620ba
diff --git a/.kitchen.yml b/.kitchen.yml
index ed53d70..c846164 100644
--- a/.kitchen.yml
+++ b/.kitchen.yml
@@ -29,9 +29,9 @@
   sudo: true
 
 platforms:
-  - name: <%=ENV['PLATFORM'] || 'ubuntu-xenial'%>
+  - name: <%=ENV['PLATFORM'] || 'ubuntu-xenial-2017.7'%>
     driver_config:
-      image: <%=ENV['PLATFORM'] || 'trevorj/salty-whales:xenial'%>
+      image: <%=ENV['PLATFORM'] || 'trevorj/salty-whales:xenial-2017.7'%>
       platform: ubuntu
 
 suites: