Fix Object#timeout deprecation warning in Ruby 2.3

Fix the following warning by explicitly including the Timeout module, rather
than relying on Object#timeout:

lib/rerun/runner.rb:254:in `stop': Object#timeout is deprecated,
use Timeout.timeout instead.

lib/rerun/runner.rb

@@ -13,6 +13,7 @@ def self.keep_running(cmd, options)
     end
 
     include System
+    include ::Timeout
 
     def initialize(run_command, options = {})
       @run_command, @options = run_command, options