Support EXTLIB=true bundle exec rake spec

Defaults to using active_support. This are the
steps to get the specs to run:

1) Getting bundler

  gem install bundler # tested with 0.9.12

2) Running specs

  # active_support
  bundle install
  ADAPTERS=mysql bundle exec rake spec

  # extlib
  bundle install --without quality
  EXTLIB=true ADAPTERS=mysql bundle exec rake spec

Gemfile

@@ -53,8 +53,11 @@ group :runtime do
   # (which probably is not going to happen anytime soon).
   #
 
-  gem 'activesupport', '~> 3.0.0.beta1', :git => 'git://github.com/rails/rails.git', :require => nil
+if ENV['EXTLIB']
   gem 'extlib',        '~> 0.9.15',      :git => 'git://github.com/datamapper/extlib.git'
+else
+  gem 'activesupport', '~> 3.0.0.beta1', :git => 'git://github.com/rails/rails.git', :require => nil
+end
   gem 'addressable',   '~> 2.1'
 
 end