Gemfile in instrumental_agent-0.13.1 vs Gemfile in instrumental_agent-0.13.2

@@ -1,8 +1,8 @@
 source "https://rubygems.org"
 
 gemspec
-
-if RUBY_VERSION < "1.9"
+ruby_engine = defined?(RUBY_ENGINE) && RUBY_ENGINE
+if RUBY_VERSION < "1.9" && !%w{jruby rbx}.include?(ruby_engine)
   # Built and installed via ext/mkrf_conf.rb
   gem 'system_timer', '~> 1.2'
 end