Use released version of test-unit-ruby-core

Author: nobu

Gemfile

@@ -1,5 +1,8 @@
 source "https://rubygems.org"
 
 gem "rake"
-gem "rake-compiler"
-gem "test-unit"
+group :development do
+  gem "rake-compiler"
+  gem "test-unit"
+  gem "test-unit-ruby-core"
+end

Rakefile

@@ -43,7 +43,7 @@ task :check do
     gem = "pkg/digest-#{version}#{"-java" if RUBY_ENGINE == "jruby"}.gem"
     File.size?(gem) or abort "gem not built!"
 
-    require_relative "test/lib/envutil"
+    require "envutil"
 
     require 'tmpdir'
     status = Dir.mktmpdir do |tmpdir|