Add test-unit constraints to the Travis Gemfile.

Author: pat

.travis/Gemfile

@@ -9,3 +9,9 @@ when "synchrony"
   gem "hiredis"
   gem "em-synchrony"
 end
+
+if RUBY_VERSION.to_f < 1.9
+  gem 'test-unit', '3.1.5'
+else
+  gem 'test-unit', '>= 3.2.5'
+end