Support Ruby 3.1

One of the [changes](https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/)
in Ruby 3.1 is that `matrix` is no longer a default gem. As such, we
need to add it as a dependency to our gemspec for compatibility with
Ruby 3.1.

Author: mkasberg

.github/workflows/ci.yml

@@ -29,10 +29,9 @@ jobs:
           - label: Ruby 3.0
             ruby_version: "3.0"
             gemfile: Gemfile
-          # classifier-reborn requires changes to work with Ruby 3.1.
-          # - label: Ruby 3.1
-          #   ruby_version: "3.1"
-          #   gemfile: Gemfile
+          - label: Ruby 3.1
+            ruby_version: "3.1"
+            gemfile: Gemfile
           - label: JRuby 9.3.4.0
             ruby_version: "jruby-9.3.4.0"
             gemfile: Gemfile-jruby

classifier-reborn-jruby.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |s|
   s.extra_rdoc_files = %w[README.markdown LICENSE]
 
   s.add_runtime_dependency('jruby-stemmer-other', '~> 0.0.2')
+  s.add_runtime_dependency('matrix', '~> 0.4')
 
   s.add_development_dependency('minitest')
   s.add_development_dependency('minitest-reporters')

classifier-reborn.gemspec

@@ -27,6 +27,7 @@ Gem::Specification.new do |s|
   s.extra_rdoc_files = %w[README.markdown LICENSE]
 
   s.add_runtime_dependency('fast-stemmer', '~> 1.0')
+  s.add_runtime_dependency('matrix', '~> 0.4')
 
   s.add_development_dependency('minitest')
   s.add_development_dependency('minitest-reporters')