Kill the test-unit gem dependency for testing on 1.9

Minitest that comes with Ruby 1.9 has a compatibility layer that allows us to
keep using it as Test::Unit, so we don't need to rely on external gems.

Besides that, with the test-unit gem we had two breaking tests that are now green.

Gemfile

@@ -3,11 +3,6 @@ source "http://rubygems.org"
 gem "rails", "~> 3.0.0"
 
 group :test do
-  gem "mocha", :require => false
-
-  if RUBY_VERSION < "1.9"
-    gem "ruby-debug", :require => false
-  else
-    gem "test-unit", :require => false
-  end
+  gem "mocha",      :require => false
+  gem "ruby-debug", :require => false, :platform => :ruby_18
 end