Fix std libs test for Ruby 2.5

mathn.rb is removed from stdlib

https://github.com/ruby/ruby/blob/v2_5_0_preview1/NEWS#stdlib-compatibility-issues-excluding-feature-bug-fixes

test/tests/ruby-standard-libs/container.rb

@@ -103,6 +103,11 @@ if defined?(RUBY_ENGINE) && RUBY_ENGINE == 'jruby'
 	stdlib.delete('mkmf')
 	stdlib.delete('objspace')
 	stdlib.delete('sdbm')
+else
+	require 'rubygems/version'
+	if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('2.5')
+		stdlib.delete('mathn')
+	end
 end
 
 result = 'ok'