Use a monotonic time for Rack::Runtime, if available

`Time.now` is prone to inaccuracies if the system time changes during the request. This could be due to DST, NTP, etc. Using a monotonic clock (available in Ruby 2.1+ on certain platforms) avoids this problem.

See [`Process.clock_gettime`’s docs](http://ruby-doc.org/core-2.1.5/Process.html#method-c-clock_gettime) for more information.