Add Salt 2018.3 tests

Change-Id: I3fe9e15a42b12265524403b466c5016d1de86e93

.kitchen.yml

 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:

.travis.yml

   - bundle install
 
 env:
-  - PLATFORM=epcim/salt-formulas:saltstack-ubuntu-xenial-salt-2016.3 SUITE=iptables-server
-  - PLATFORM=epcim/salt-formulas:saltstack-ubuntu-xenial-salt-2017.7 SUITE=iptables-server
-#  - PLATFORM=epcim/salt-formulas:saltstack-ubuntu-bionic-salt-2017.7 SUITE=iptables-server
+  - PLATFORM=epcim/salt:saltstack-ubuntu-xenial-salt-2016.3 SUITE=iptables-server
+  - PLATFORM=epcim/salt:saltstack-ubuntu-xenial-salt-2017.7 SUITE=iptables-server
+  - PLATFORM=epcim/salt:saltstack-ubuntu-xenial-salt-2018.3 SUITE=iptables-server
+#  - PLATFORM=epcim/salt:saltstack-ubuntu-bionic-salt-2017.7 SUITE=iptables-server
+#  - PLATFORM=epcim/salt:saltstack-ubuntu-bionic-salt-2018.3 SUITE=iptables-server
 
 before_script:
   - set -o pipefail