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

SaltStack provides "versioned" repositories, this commit add a way
to set which release of salt to use.
It adds a pillar "salt:release" which can be set to a specific release
(ex: 2016.11). This release is then used to configure properly the
repositories URLs for Debian/Ubuntu/RedHat.
The default behavior is to point to 'latest', retaining the previous
behavior if the "salt:release" pillar is not set.

issue #180

* 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