Using Kernel.exit to avoid error messages when exiting irb

Resolves #285

On the latest version of simplecov, errors were being throw on console exit:

../.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/irb.rb:83:in `throw': uncaught throw :IRB_EXIT (ArgumentError)
	from ../.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/irb.rb:83:in `irb_exit'

This addresses this problem by calling Kernel.exit directly, rather than irb's exit. Not sure if this is related to issue #281, but nonethless getting rid of this annoying error message would be great! :)