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/pkgs_spec.rb

@@ -0,0 +1,24 @@
+case os[:name]
+when 'centos'
+  version = '2018.3.4-1.el6'
+when 'debian', 'ubuntu'
+  version = '2018.3.4+ds-1'
+when 'opensuse'
+  version = '2018.3.0-42.1'
+when 'fedora'
+  version = '2018.3.0-1.fc28'
+end
+
+control 'salt packages' do
+  title 'should be installed'
+
+  describe package('salt-master') do
+    it { should be_installed }
+    its('version') { should eq version }
+  end
+
+  describe package('salt-minion') do
+    it { should be_installed }
+    its('version') { should eq version }
+  end
+end

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