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.
Merge pull request #762 from amarshall/monotonic-runtime

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