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/2019-2/pkgs_spec.rb

@@ -0,0 +1,22 @@
+case os[:name]
+when 'centos'
+  version = '2019.2.0-1.el7'
+when 'fedora'
+  version = '2019.2.0-1.fc29'
+when 'debian', 'ubuntu'
+  version = '2019.2.0+ds-1'
+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