Add a notes regarding ruby >= 1.9 requirement for BACKGROUND

Author: cleishm

README.markdown

@@ -267,9 +267,10 @@ worker process.  Use the PIDFILE option for easy access to the PID:
 
 ### Running in the background
 
-There are scenarios where it's helpful for the resque worker to run
-itself in the background (usually in combination with PIDFILE).  Use the
-BACKGROUND option so that rake will return as soon as the worker is started.
+(Only supported with ruby >= 1.9). There are scenarios where it's helpful for
+the resque worker to run itself in the background (usually in combination with
+PIDFILE).  Use the BACKGROUND option so that rake will return as soon as the
+worker is started.
 
     $ PIDFILE=./resque.pid BACKGROUND=yes QUEUE=file_serve \
         rake environment resque:work

lib/resque/tasks.rb

@@ -19,6 +19,9 @@
     end
 
     if ENV['BACKGROUND']
+      unless Process.respond_to?('daemon')
+          abort "env var BACKGROUND is set, which requires ruby >= 1.9"
+      end
       Process.daemon(true)
     end