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

salt/pkgrepo/redhat/init.sls

@@ -1,5 +1,6 @@
 {% from "salt/map.jinja" import salt_settings with context %}
 
+{%- if grains['os']|lower not in ('amazon', 'fedora', 'suse',) %}
 saltstack-pkgrepo:
   pkgrepo.managed:
     - humanname: SaltStack repo for RHEL/CentOS $releasever
@@ -7,3 +8,4 @@ saltstack-pkgrepo:
     - enabled: 1
     - gpgcheck: 1
     - gpgkey: {{ salt_settings.key_url }}
+{% endif %}