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
* Install pygit from package
* Add Official Salt ZeroMQ 4 COPR repository
Note that Salt itself is assumed to already to be available to the system via yum, via EPEL for example