prevent circular loading errors / always loading

In any non-bundled project minitest-rg is always getting loaded and then blows up -> change it to be opt-in, so you drop `require 'minitest/rg'` and everything works

```
ruby -Itest /Users/mgrosser/Code/tools/minitest/test/minitest/test_minitest_spec.rb -n /setup_teardown_behavior/
/opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-rg-5.0.0.20131113213436/lib/minitest/rg.rb:4:in `<top (required)>': uninitialized constant Minitest::RG (NameError)
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:118:in `require'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:118:in `rescue in require'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:124:in `require'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-rg-5.0.0.20131113213436/lib/minitest/rg_plugin.rb:2:in `<top (required)>'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:114:in `require'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:114:in `require'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-5.2.2/lib/minitest.rb:90:in `block in load_plugins'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-5.2.2/lib/minitest.rb:84:in `each'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-5.2.2/lib/minitest.rb:84:in `load_plugins'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-5.2.2/lib/minitest.rb:113:in `run'
	from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/minitest-5.2.2/lib/minitest.rb:55:in `block in autorun'
```