Remove Arel's git repo from Gemfile and use system_timer just on 1.8.x

Gemfile

 path File.dirname(__FILE__)
 source 'http://rubygems.org'
 
-gem "arel", :git => "git://github.com/rails/arel.git"
 gem "rails", "3.0.0.beta1"
 
 gem "rake",  ">= 0.8.7"
 gem "mocha", ">= 0.9.8"
 
 group :mri do
-  gem "system_timer"
   if RUBY_VERSION < '1.9'
+    gem "system_timer"
     gem "ruby-debug", ">= 0.10.3"
   end
 end