Remove facter dependency

[swrobel]

• Facter required a native extension, and was used to determine Linux distro - use /etc/os-release to get that info much more simply
• Add a new spec to test custom distro detection
• Remove Mint distro as it was just a direct descendant of Ubuntu with no overrides
• Remove dependency on git from gemspec - use pure ruby to get file lists instead
• Use docker on Travis CI to test all supported distros (only testing distro detection for now)
• Only run full spec suite using latest ruby