Reconsider required Ruby version

[QuintinWillison]

Our root readme for this SDK states:

This SDK supports Ruby 1.9.3+.

But, as AndyNicks has pointed out (internal Slack message), Ruby 1 is no longer used (2019 survey!).

We need to decide on our new baseline version of Ruby and then make the changes to this SDK to make that happen.

Relates to #253

┆Issue is synchronized with this Jira Uncategorised by Unito