Drop support for Ruby 2.3 and 2.4

[denisdefreyne]

As per the compatibility policy:

Cri is guaranteed to be supported on any officially supported Ruby version, as well as the version of Ruby that comes by default on

• the last two Ubuntu LTS releases
• the last two major macOS releases

The last two Ubuntu LTS releases and the the two major macOS releases come with at least Ruby 2.5, so we can drop support for Ruby 2.3 and 2.4.

[denisdefreyne]

CC @scotje @binford2k @donoghuc — this will likely be relevant to you. The next release of Cri will drop Ruby 2.3 and 2.4 support. (Also see #95, which adds the compatibility policy.)

The version number will likely be a patch number bump rather than a major number bump, i.e. from 2.15.10 to 2.15.11 rather than to 3.0.0.