Fix test failures on Ruby 1.8.7.

https://travis-ci.org/tzinfo/tzinfo/jobs/655518112

test/test_utils.rb

@@ -116,7 +116,7 @@ def assert_array_same_items(expected, actual, msg = nil)
   end
 
   def assert_sub_process_returns(expected_lines, code, extra_load_path = [], required = ['tzinfo'])
-    if RUBY_ENGINE == 'jruby' && JRUBY_VERSION.start_with?('9.0.') && RbConfig::CONFIG['host_os'] =~ /mswin/
+    if defined?(RUBY_ENGINE) && RUBY_ENGINE == 'jruby' && JRUBY_VERSION.start_with?('9.0.') && RbConfig::CONFIG['host_os'] =~ /mswin/
       skip('JRuby 9.0 on Windows cannot handle writing to the IO instance returned by popen')
     end