Make tests pass on newer Ruby versions

[olleolleolle]

Fixes #31

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Ruby 3.1: Some actionable output.

DEPRECATED: Use assert_nil if expecting nil from /home/runner/work/kleisli/kleisli/test/kleisli/maybe_test.rb:9. This will fail in Minitest 6.
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:33: warning: constant ::Fixnum is deprecated
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:51: warning: constant ::Fixnum is deprecated
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:23: warning: constant ::Fixnum is deprecated
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:7: warning: constant ::Fixnum is deprecated
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:14: warning: constant ::Fixnum is deprecated
/home/runner/work/kleisli/kleisli/test/kleisli/composition_test.rb:42: warning: constant ::Fixnum is deprecated

Ruby 3.2: needs a newer Rake

NoMethodError: undefined method `=~' for #<Proc:0x00007f6b8ff27bd0 /home/runner/work/kleisli/kleisli/vendor/bundle/ruby/3.2.0/gems/rake-10.5.0/lib/rake/application.rb:393 (lambda)>

(and the 3.1 output had become errors)

[olleolleolle]

👋 @txus There!