Update the testing matrix to cover most distro+salt+python combinations

Use py3 for latest salt version
Use Python 2 on old distros
Update Gemfile
Updated centos-6 run_command, as it does not use systemd
Added tests to verify correct package versions for all cases tested

test/integration/2018-3/service_spec.rb

@@ -0,0 +1,14 @@
+control 'salt services' do
+  title 'should be running'
+
+  describe service('salt-master') do
+    it { should be_enabled }
+    it { should be_running }
+  end
+
+  describe service('salt-minion') do
+    it { should be_enabled }
+    it { should be_running }
+  end
+
+end