* lib/test/unit/test-unit.gemspec: Add minitest < 5.0.0 dependency

* tool/rbinstall.rb: Add empty implementations for `add_dependency`,
  `add_runtime_dependency`, `add_development_dependency` for
  Gem::Specification.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45032 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* lib/test/unit.rb: Requires minitest < 5.0.0 if Gem is available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45033 b2dd03c8-39d4-4d8f-98ff-823fe69b080e