Fix ruby 2.1 deprecation warning for .ago

Ruby 2.1 has deprecated .ago and .until for Fixnum causing a lot of warnings if the Timeoutable module is enabled.

DEPRECATION WARNING: Calling #ago or #until on a number (e.g. 5.ago) is deprecated and will be removed in the future, use 5.seconds.ago instead. (called from service at /Users/joe/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/webrick/httpserver.rb:138)