fix to rescue when test-all runs without installation

https://api.travis-ci.org/jobs/23564212/log.txt?deansi=true

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45686 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/test/unit.rb

@@ -1,4 +1,7 @@
-gem 'minitest', '< 5.0.0' if defined? Gem
+begin
+  gem 'minitest', '< 5.0.0' if defined? Gem
+rescue Gem::LoadError
+end
 require 'minitest/unit'
 require 'test/unit/assertions'
 require 'test/unit/testcase'