Add Salt 2018.3 tests and fix readme

Change-Id: I6b5be381afc4d0f4f6c95f302f3eec1068648c57
diff --git a/.kitchen.yml b/.kitchen.yml
index 7af31ff..050f800 100644
--- a/.kitchen.yml
+++ b/.kitchen.yml
@@ -36,7 +36,7 @@
 platforms:
   - name: <%=ENV['PLATFORM'] ||  'saltstack-ubuntu-xenial-salt-stable' %>
     driver_config:
-      image: <%=ENV['PLATFORM'] || 'epcim/salt-formulas:saltstack-ubuntu-xenial-salt-stable'%>
+      image: <%=ENV['PLATFORM'] || 'epcim/salt:saltstack-ubuntu-xenial-salt-stable'%>
       platform: ubuntu
 
 suites: