Remove support for EOL Ruby versions

[bensheldon]

This gem currently targets Ruby versions 1.9.3 - 2.1. These versions have been end-of-lifed as of 2015 - 2017, respectively.

Currently maintained Ruby versions are 2.6 - 3.0.

Forward-looking changes, that likely would be backwards incompatible, would be Ruby 2 style hashes and kwargs.