Merge pull request #4237 from castlerock/fiber_available_in_ruby19

fiber is available in ruby19 condition removed

actionpack/lib/action_view/renderer/streaming_template_renderer.rb

 # 1.9 ships with Fibers but we need to require the extra
 # methods explicitly. We only load those extra methods if
 # Fiber is available in the first place.
-require 'fiber' if defined?(Fiber)
+require 'fiber'
 
 module ActionView
   # == TODO