Skip unsupported rubygems update on EOL rubies
Skip unsupported rubygems update on EOL rubies

Rubygems has dropped support for Ruby < 2.3. As a result, we can no
longer run `gem update --system` on those version of Ruby.

This commit also updates the Travis config to use the latest Ruby patch
versions.

mattbrictson/airbrussh,mattbrictson/airbrussh