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/default/pkgs_spec.rb

@@ -1,11 +0,0 @@
-control 'salt packages' do
-  title 'should be installed'
-
-  describe package('salt-master') do
-    it { should be_installed }
-  end
-
-  describe package('salt-minion') do
-    it { should be_installed }
-  end
-end

test/integration/default/service_spec.rb

@@ -1,14 +0,0 @@
-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