Enable warnings in irb as well using -W

gistfile1.rb

dczarnecki-agora:~ dczarnecki$ ruby -v
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0]
dczarnecki-agora:~ dczarnecki$ irb -W
ruby-1.9.2-p180 :001 > def hello
ruby-1.9.2-p180 :002?>   p 'world'
ruby-1.9.2-p180 :003?>   end
 => nil 
ruby-1.9.2-p180 :004 > hello
"world"
 => "world" 
ruby-1.9.2-p180 :005 > def hello
ruby-1.9.2-p180 :006?>   p 'hello world'
ruby-1.9.2-p180 :007?>   end
(irb):5: warning: method redefined; discarding old hello
(irb):1: warning: previous definition of hello was here
 => nil 
ruby-1.9.2-p180 :008 > hello
"hello world"
 => "hello world" 
ruby-1.9.2-p180 :009 >