Add logger dependency to fix warning on Ruby 3.4

[mattbrictson]

When using childprocess on Ruby 3.4.0.dev, the following warning is printed:

lib/childprocess.rb:7: warning: logger was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0. Add logger to your Gemfile or gemspec.

This PR fixes the warning by adding the logger gem as a gemspec dependency.

I chose ~> 1.5 as the requirement because 1.5.0 is the most recent version that supports Ruby 2.4.

I also added Ruby head to the CI matrix.

[mattbrictson]

The CI failures are hopefully temporary, due to a broken release of the term-ansicolor gem: flori/term-ansicolor#41

[sds]

Thanks!