travis: test with strict_variables on puppet 4

.travis.yml

@@ -17,6 +17,6 @@ matrix:
     - rvm: 2.1.0
       env: PUPPET_GEM_VERSION="~> 3.0"
     - rvm: 2.1.0
-      env: PUPPET_GEM_VERSION="~> 4.0"
+      env: PUPPET_GEM_VERSION="~> 4.0" STRICT_VARIABLES="yes"
 notifications:
   email: false