Update .travis.yml and .kitchen.yml files

Change-Id: I7731a2e79e5791f5eb1b13970e6b7ce6e0d89a8b
diff --git a/.kitchen.yml b/.kitchen.yml
index 1500184..a804e9d 100644
--- a/.kitchen.yml
+++ b/.kitchen.yml
@@ -32,14 +32,9 @@
   sudo: true
 
 platforms:
-  - name: ubuntu-trusty
+  - name: <%=ENV['PLATFORM'] || 'ubuntu-xenial'%>
     driver_config:
-      image: trevorj/salty-whales:trusty
-      platform: ubuntu
-
-  - name: ubuntu-xenial
-    driver_config:
-      image: trevorj/salty-whales:xenial
+      image: <%=ENV['PLATFORM'] || 'trevorj/salty-whales:xenial'%>
       platform: ubuntu
 
 suites: