Allow devs to opt out of test warnings

Author: bf4

Rakefile

@@ -43,7 +43,8 @@ Rake::TestTask.new do |t|
   t.libs << 'test'
   t.libs << 'lib'
   t.test_files = FileList['test/**/*_test.rb']
-  t.ruby_opts = ['-w -r./test/test_helper.rb']
+  t.ruby_opts = ['-r./test/test_helper.rb']
+  t.ruby_opts << ' -w' unless ENV['NO_WARN'] == 'true'
   t.verbose = true
 end