Specify `required_ruby_version`

[pboling]

In order to do this, and not issue a major version bump, it would be ideal to first determine the oldest version that currently works with the gem, and specify that as the spec.required_ruby_version. This will have the effect of explicitly "dropping" everything that already doesn't work.

Based on #2 it would seem that this gem originally supported Ruby 1.8, and it seems like that may be the best initial value of required_ruby_version.

If you do want to do a major version bump later you could drop older Rubies at that time.

Note that require_relative wasn't added until Ruby 2.1, so it wouldn't be useable until the required ruby version has surpassed that number.